assignment on lymphatic system

21.1 Anatomy of the Lymphatic and Immune Systems

Learning objectives.

By the end of this section, you will be able to:

• Describe the structure and function of the lymphatic tissue (lymph fluid, vessels, ducts, and organs)
• Describe the structure and function of the primary and secondary lymphatic organs
• Discuss the cells of the immune system, how they function, and their relationship with the lymphatic system

The immune system is the complex collection of cells and organs that destroys or neutralizes pathogens that would otherwise cause disease or death. The lymphatic system, for most people, is associated with the immune system to such a degree that the two systems are virtually indistinguishable. The lymphatic system is the system of vessels, cells, and organs that carries excess fluids to the bloodstream and filters pathogens from the blood. The swelling of lymph nodes during an infection and the transport of lymphocytes via the lymphatic vessels are but two examples of the many connections between these critical organ systems.

Functions of the Lymphatic System

A major function of the lymphatic system is to drain body fluids and return them to the bloodstream. Blood pressure causes leakage of fluid from the capillaries, resulting in the accumulation of fluid in the interstitial space—that is, spaces between individual cells in the tissues. In humans, 20 liters of plasma is released into the interstitial space of the tissues each day due to capillary filtration. Once this filtrate is out of the bloodstream and in the tissue spaces, it is referred to as interstitial fluid. Of this, 17 liters is reabsorbed directly by the blood vessels. But what happens to the remaining three liters? This is where the lymphatic system comes into play. It drains the excess fluid and empties it back into the bloodstream via a series of vessels, trunks, and ducts. Lymph is the term used to describe interstitial fluid once it has entered the lymphatic system. When the lymphatic system is damaged in some way, such as by being blocked by cancer cells or destroyed by injury, protein-rich interstitial fluid accumulates (sometimes “backs up” from the lymph vessels) in the tissue spaces. This inappropriate accumulation of fluid referred to as lymphedema may lead to serious medical consequences.

As the vertebrate immune system evolved, the network of lymphatic vessels became convenient avenues for transporting the cells of the immune system. Additionally, the transport of dietary lipids and fat-soluble vitamins absorbed in the gut uses this system.

Cells of the immune system not only use lymphatic vessels to make their way from interstitial spaces back into the circulation, but they also use lymph nodes as major staging areas for the development of critical immune responses. A lymph node is one of the small, bean-shaped organs located throughout the lymphatic system.

External Website

Visit this website for an overview of the lymphatic system. What are the three main components of the lymphatic system?

Structure of the Lymphatic System

The lymphatic vessels begin as open-ended capillaries, which feed into larger and larger lymphatic vessels, and eventually empty into the bloodstream by a series of ducts. Along the way, the lymph travels through the lymph nodes, which are commonly found near the groin, armpits, neck, chest, and abdomen. Humans have about 500–600 lymph nodes throughout the body ( Figure 21.1.1 ).

A major distinction between the lymphatic and cardiovascular systems in humans is that lymph is not actively pumped by the heart, but is forced through the vessels by the movements of the body, the contraction of skeletal muscles during body movements, and breathing. One-way valves (semi-lunar valves) in lymphatic vessels keep the lymph moving toward the heart. Lymph flows from the lymphatic capillaries, through lymphatic vessels, and then is dumped into the circulatory system via the lymphatic ducts located at the junction of the jugular and subclavian veins in the neck.

Lymphatic Capillaries

Lymphatic capillaries , also called the terminal lymphatics, are vessels where interstitial fluid enters the lymphatic system to become lymph fluid. Located in almost every tissue in the body, these vessels are interlaced among the arterioles and venules of the circulatory system in the soft connective tissues of the body ( Figure 21.1.2 ). Exceptions are the central nervous system, bone marrow, bones, teeth, and the cornea of the eye, which do not contain lymph vessels.

Lymphatic capillaries are formed by a one cell-thick layer of endothelial cells and represent the open end of the system, allowing interstitial fluid to flow into them via overlapping cells (see Figure 21.1.2 ). When interstitial pressure is low, the endothelial flaps close to prevent “backflow.” As interstitial pressure increases, the spaces between the cells open up, allowing the fluid to enter. Entry of fluid into lymphatic capillaries is also enabled by the collagen filaments that anchor the capillaries to surrounding structures. As interstitial pressure increases, the filaments pull on the endothelial cell flaps, opening up them even further to allow easy entry of fluid.

In the small intestine, lymphatic capillaries called lacteals are critical for the transport of dietary lipids and lipid-soluble vitamins to the bloodstream. In the small intestine, dietary triglycerides combine with other lipids and proteins, and enter the lacteals to form a milky fluid called chyle . The chyle then travels through the lymphatic system, eventually entering the liver and then the bloodstream.

Larger Lymphatic Vessels, Trunks, and Ducts

The lymphatic capillaries empty into larger lymphatic vessels, which are similar to veins in terms of their three-tunic structure and the presence of valves. These one-way valves are located fairly close to one another, and each one causes a bulge in the lymphatic vessel, giving the vessels a beaded appearance (see Figure 21.1.2 ).

The superficial and deep lymphatics eventually merge to form larger lymphatic vessels known as lymphatic trunks . On the right side of the body, the right sides of the head, thorax, and right upper limb drain lymph fluid into the right subclavian vein via the right lymphatic duct ( Figure 21.1.3 ). On the left side of the body, the remaining portions of the body drain into the larger thoracic duct, which drains into the left subclavian vein. The thoracic duct itself begins just beneath the diaphragm in the cisterna chyli , a sac-like chamber that receives lymph from the lower abdomen, pelvis, and lower limbs by way of the left and right lumbar trunks and the intestinal trunk.

The overall drainage system of the body is asymmetrical (see Figure 21.1.3 ). The right lymphatic duct receives lymph from only the upper right side of the body. The lymph from the rest of the body enters the bloodstream through the thoracic duct via all the remaining lymphatic trunks. In general, lymphatic vessels of the subcutaneous tissues of the skin, that is, the superficial lymphatics, follow the same routes as veins, whereas the deep lymphatic vessels of the viscera generally follow the paths of arteries.

The Organization of Immune Function

The immune system is a collection of barriers, cells, and soluble proteins that interact and communicate with each other in extraordinarily complex ways. The modern model of immune function is organized into three phases based on the timing of their effects. The three temporal phases consist of the following:

• Barrier defenses such as the skin and mucous membranes, which act instantaneously to prevent pathogenic invasion into the body tissues
• The rapid but nonspecific innate immune response , which consists of a variety of specialized cells and soluble factors
• The slower but more specific and effective adaptive immune response , which involves many cell types and soluble factors, but is primarily controlled by white blood cells (leukocytes) known as lymphocytes , which help control immune responses

The cells of the blood, including all those involved in the immune response, arise in the bone marrow via various differentiation pathways from hematopoietic stem cells ( Figure 21.1.4 ). In contrast with embryonic stem cells, hematopoietic stem cells are present throughout adulthood and allow for the continuous differentiation of blood cells to replace those lost to age or function. These cells can be divided into three classes based on function:

• Phagocytic cells, which ingest pathogens to destroy them
• Lymphocytes, which specifically coordinate the activities of adaptive immunity
• Cells containing cytoplasmic granules, which help mediate immune responses against parasites and intracellular pathogens such as viruses

Lymphocytes: B Cells, T Cells, Plasma Cells, and Natural Killer Cells

As stated above, lymphocytes are the primary cells of adaptive immune responses ( Table 21.1 ). The two basic types of lymphocytes, B cells and T cells, are identical morphologically with a large central nucleus surrounded by a thin layer of cytoplasm. They are distinguished from each other by their surface protein markers as well as by the molecules they secrete. While B cells mature in red bone marrow and T cells mature in the thymus, they both initially develop from bone marrow. T cells migrate from bone marrow to the thymus gland where they further mature. B cells and T cells are found in many parts of the body, circulating in the bloodstream and lymph, and residing in secondary lymphoid organs, including the spleen and lymph nodes, which will be described later in this section. The human body contains approximately 10 12 lymphocytes.

B cells are immune cells that function primarily by producing antibodies. An antibody is any of the group of proteins that binds specifically to pathogen-associated molecules known as antigens. An antigen is a chemical structure on the surface of a pathogen that binds to T or B lymphocyte antigen receptors. Once activated by binding to antigen, B cells differentiate into cells that secrete a soluble form of their surface antibodies. These activated B cells are known as plasma cells.

The T cell , on the other hand, does not secrete antibody but performs a variety of functions in the adaptive immune response. Different T cell types have the ability to either secrete soluble factors that communicate with other cells of the adaptive immune response or destroy cells infected with intracellular pathogens. The roles of T and B lymphocytes in the adaptive immune response will be discussed further in this chapter.

Plasma Cells

Another type of lymphocyte of importance is the plasma cell. A plasma cell is a B cell that has differentiated in response to antigen binding, and has thereby gained the ability to secrete soluble antibodies. These cells differ in morphology from standard B and T cells in that they contain a large amount of cytoplasm packed with the protein-synthesizing machinery known as rough endoplasmic reticulum.

Natural Killer Cells

A fourth important lymphocyte is the natural killer cell, a participant in the innate immune response. A natural killer cell (NK) is a circulating blood cell that contains cytotoxic (cell-killing) granules in its extensive cytoplasm. It shares this mechanism with the cytotoxic T cells of the adaptive immune response. NK cells are among the body’s first lines of defense against viruses and certain types of cancer.

Visit this website to learn about the many different cell types in the immune system and their very specialized jobs. What is the role of the dendritic cell in an HIV infection?

Primary Lymphoid Organs and Lymphocyte Development

Understanding the differentiation and development of B and T cells is critical to the understanding of the adaptive immune response. It is through this process that the body (ideally) learns to destroy only pathogens and leaves the body’s own cells relatively intact. The primary lymphoid organs are the bone marrow and thymus gland. The lymphoid organs are where lymphocytes mature, proliferate, and are selected, which enables them to attack pathogens without harming the cells of the body.

Bone Marrow

In the embryo, blood cells are made in the yolk sac. As development proceeds, this function is taken over by the spleen, lymph nodes, and liver. Later, the bone marrow takes over most hematopoietic functions, although the final stages of the differentiation of some cells may take place in other organs. The red bone marrow is a loose collection of cells where hematopoiesis occurs, and the yellow bone marrow is a site of energy storage, which consists largely of fat cells ( Figure 21.1.5 ). The B cell undergoes nearly all of its development in the red bone marrow, whereas the immature T cell, called a thymocyte , leaves the bone marrow and matures largely in the thymus gland.

The thymus gland is a bilobed organ found in the space between the sternum and the aorta of the heart ( Figure 21.1.6 ). Connective tissue holds the lobes closely together but also separates them and forms a capsule.

View the University of Michigan WebScope at http://141.214.65.171/Histology/Lymphatic%20System/140_HISTO_40X.svs/view.apml to explore the tissue sample in greater detail.

The connective tissue capsule further divides the thymus into lobules via extensions called trabeculae. The outer region of the organ is known as the cortex and contains large numbers of thymocytes with some epithelial cells, macrophages, and dendritic cells (two types of phagocytic cells that are derived from monocytes). The cortex is densely packed so it stains more intensely than the rest of the thymus (see Figure 21.1.6 ). The medulla, where thymocytes migrate before leaving the thymus, contains a less dense collection of thymocytes, epithelial cells, and dendritic cells.

By the year 2050, 25 percent of the population of the United States will be 60 years of age or older. The CDC estimates that 80 percent of those 60 years and older have one or more chronic disease associated with deficiencies of the immune systems. This loss of immune function with age is called immunosenescence. To treat this growing population, medical professionals must better understand the aging process. One major cause of age-related immune deficiencies is thymic involution, the shrinking of the thymus gland that begins at birth, at a rate of about three percent tissue loss per year, and continues until 35–45 years of age, when the rate declines to about one percent loss per year for the rest of one’s life. At that pace, the total loss of thymic epithelial tissue and thymocytes would occur at about 120 years of age. Thus, this age is a theoretical limit to a healthy human lifespan.

Thymic involution has been observed in all vertebrate species that have a thymus gland. Animal studies have shown that transplanted thymic grafts between inbred strains of mice involuted according to the age of the donor and not of the recipient, implying the process is genetically programmed. There is evidence that the thymic microenvironment, so vital to the development of naïve T cells, loses thymic epithelial cells according to the decreasing expression of the FOXN1 gene with age.

It is also known that thymic involution can be altered by hormone levels. Sex hormones such as estrogen and testosterone enhance involution, and the hormonal changes in pregnant women cause a temporary thymic involution that reverses itself, when the size of the thymus and its hormone levels return to normal, usually after lactation ceases. What does all this tell us? Can we reverse immunosenescence, or at least slow it down? The potential is there for using thymic transplants from younger donors to keep thymic output of naïve T cells high. Gene therapies that target gene expression are also seen as future possibilities. The more we learn through immunosenescence research, the more opportunities there will be to develop therapies, even though these therapies will likely take decades to develop. The ultimate goal is for everyone to live and be healthy longer, but there may be limits to immortality imposed by our genes and hormones.

Secondary Lymphoid Organs and their Roles in Active Immune Responses

Lymphocytes develop and mature in the primary lymphoid organs, but they mount immune responses from the secondary lymphoid organs . A naïve lymphocyte is one that has left the primary organ and entered a secondary lymphoid organ. Naïve lymphocytes are fully functional immunologically, but have yet to encounter an antigen to respond to. In addition to circulating in the blood and lymph, lymphocytes concentrate in secondary lymphoid organs, which include the lymph nodes, spleen, and lymphoid nodules. All of these tissues have many features in common, including the following:

• The presence of lymphoid follicles, the sites of the formation of lymphocytes, with specific B cell-rich and T cell-rich areas
• An internal structure of reticular fibers with associated fixed macrophages
• Germinal centers , which are the sites of rapidly dividing B lymphocytes and plasma cells, with the exception of the spleen
• Specialized post-capillary vessels known as high endothelial venules ; the cells lining these venules are thicker and more columnar than normal endothelial cells, which allow cells from the blood to directly enter these tissues

Lymph Nodes

Lymph nodes function to remove debris and pathogens from the lymph, and are thus sometimes referred to as the “filters of the lymph” ( Figure 21.1.7 ). Any bacteria that infect the interstitial fluid are taken up by the lymphatic capillaries and transported to a regional lymph node. Dendritic cells and macrophages within this organ internalize and kill many of the pathogens that pass through, thereby removing them from the body. The lymph node is also the site of adaptive immune responses mediated by T cells, B cells, and accessory cells of the adaptive immune system. Like the thymus, the bean-shaped lymph nodes are surrounded by a tough capsule of connective tissue and are separated into compartments by trabeculae, the extensions of the capsule. In addition to the structure provided by the capsule and trabeculae, the structural support of the lymph node is provided by a series of reticular fibers laid down by fibroblasts.

View the University of Michigan WebScope at http://141.214.65.171/Histology/Lymphatic%20System/142_HISTO_40X.svs/view.apml to explore the tissue sample in greater detail.

The major routes into the lymph node are via afferent lymphatic vessels (see Figure 21.1.7 ). Cells and lymph fluid that leave the lymph node may do so by another set of vessels known as the efferent lymphatic vessels . Lymph enters the lymph node via the subcapsular sinus, which is occupied by dendritic cells, macrophages, and reticular fibers. Within the cortex of the lymph node are lymphoid follicles, which consist of germinal centers of rapidly dividing B cells surrounded by a layer of T cells and other accessory cells. As the lymph continues to flow through the node, it enters the medulla, which consists of medullary cords of B cells and plasma cells, and the medullary sinuses where the lymph collects before leaving the node via the efferent lymphatic vessels.

In addition to the lymph nodes, the spleen is a major secondary lymphoid organ ( Figure 21.1.8 ). It is about 12 cm (5 in) long and is attached to the lateral border of the stomach via the gastrosplenic ligament. The spleen is a fragile organ without a strong capsule, and is dark red due to its extensive vascularization. The spleen is sometimes called the “filter of the blood” because of its extensive vascularization and the presence of macrophages and dendritic cells that remove microbes and other materials from the blood, including dying red blood cells. The spleen also functions as the location of immune responses to blood-borne pathogens.

The spleen is also divided by trabeculae of connective tissue, and within each splenic nodule is an area of red pulp, consisting of mostly red blood cells, and white pulp, which resembles the lymphoid follicles of the lymph nodes. Upon entering the spleen, the splenic artery splits into several arterioles (surrounded by white pulp) and eventually into sinusoids. Blood from the capillaries subsequently collects in the venous sinuses and leaves via the splenic vein. The red pulp consists of reticular fibers with fixed macrophages attached, free macrophages, and all of the other cells typical of the blood, including some lymphocytes. The white pulp surrounds a central arteriole and consists of germinal centers of dividing B cells surrounded by T cells and accessory cells, including macrophages and dendritic cells. Thus, the red pulp primarily functions as a filtration system of the blood, using cells of the relatively nonspecific immune response, and white pulp is where adaptive T and B cell responses are mounted.

Lymphoid Nodules

The other lymphoid tissues, the lymphoid nodules , have a simpler architecture than the spleen and lymph nodes in that they consist of a dense cluster of lymphocytes without a surrounding fibrous capsule. These nodules are located in the respiratory and digestive tracts, areas routinely exposed to environmental pathogens.

Tonsils are lymphoid nodules located along the inner surface of the pharynx and are important in developing immunity to oral pathogens ( Figure 21.1.9 ). The tonsil located at the back of the throat, the pharyngeal tonsil, is sometimes referred to as the adenoid when swollen. Such swelling is an indication of an active immune response to infection. Histologically, tonsils do not contain a complete capsule, and the epithelial layer invaginates deeply into the interior of the tonsil to form tonsillar crypts. These structures, which accumulate all sorts of materials taken into the body through eating and breathing, actually “encourage” pathogens to penetrate deep into the tonsillar tissues where they are acted upon by numerous lymphoid follicles and eliminated. This seems to be the major function of tonsils—to help children’s bodies recognize, destroy, and develop immunity to common environmental pathogens so that they will be protected in their later lives. Tonsils are often removed in those children who have recurring throat infections, especially those involving the palatine tonsils on either side of the throat, whose swelling may interfere with their breathing and/or swallowing.

View the University of Michigan WebScope at http://141.214.65.171/Histology/Lymphatic%20System/138_HISTO_20X.svs/view.apml to explore the tissue sample in greater detail.

Mucosa-associated lymphoid tissue (MALT) consists of an aggregate of lymphoid follicles directly associated with the mucous membrane epithelia. MALT makes up dome-shaped structures found underlying the mucosa of the gastrointestinal tract, breast tissue, lungs, and eyes. Peyer’s patches, a type of MALT in the small intestine, are especially important for immune responses against ingested substances ( Figure 21.1.10 ). Peyer’s patches contain specialized endothelial cells called M (or microfold) cells that sample material from the intestinal lumen and transport it to nearby follicles so that adaptive immune responses to potential pathogens can be mounted.

Bronchus-associated lymphoid tissue (BALT) consists of lymphoid follicular structures with an overlying epithelial layer found along the bifurcations of the bronchi, and between bronchi and arteries. They also have the typically less-organized structure of other lymphoid nodules. These tissues, in addition to the tonsils, are effective against inhaled pathogens.

Chapter Review

The lymphatic system is a series of vessels, ducts, and trunks that remove interstitial fluid from the tissues and return it the blood. The lymphatics are also used to transport dietary lipids and cells of the immune system. Cells of the immune system all come from the hematopoietic system of the bone marrow. Primary lymphoid organs, the bone marrow and thymus gland, are the locations where lymphocytes of the adaptive immune system proliferate and mature. Secondary lymphoid organs are site in which mature lymphocytes congregate to mount immune responses. Many immune system cells use the lymphatic and circulatory systems for transport throughout the body to search for and then protect against pathogens.

Interactive Link Questions

The three main components are the lymph vessels, the lymph nodes, and the lymph.

Visit this website to learn about the many different cell types in the immune system and their very specialized jobs. What is the role of the dendritic cell in infection by HIV?

The dendritic cell transports the virus to a lymph node.

Review Questions

Critical thinking questions.

1. Describe the flow of lymph from its origins in interstitial fluid to its emptying into the venous bloodstream.

Answers for Critical Thinking Questions

• The lymph enters through lymphatic capillaries, and then into larger lymphatic vessels. The lymph can only go in one direction due to valves in the vessels. The larger lymphatics merge to form trunks that enter into the blood via lymphatic ducts.

This work, Anatomy & Physiology, is adapted from Anatomy & Physiology by OpenStax , licensed under CC BY . This edition, with revised content and artwork, is licensed under CC BY-SA except where otherwise noted.

Images, from Anatomy & Physiology by OpenStax , are licensed under CC BY except where otherwise noted.

Access the original for free at https://openstax.org/books/anatomy-and-physiology/pages/1-introduction .

Anatomy & Physiology Copyright © 2019 by Lindsay M. Biga, Staci Bronson, Sierra Dawson, Amy Harwell, Robin Hopkins, Joel Kaufmann, Mike LeMaster, Philip Matern, Katie Morrison-Graham, Kristen Oja, Devon Quick, Jon Runyeon, OSU OERU, and OpenStax is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License , except where otherwise noted.

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How the Lymphatic System Works

How does the lymphatic system work, lymphatic system organs, lymphatic diseases, lymphatic system and general health, caring for your lymphatic system, frequently asked questions.

The lymphatic system (also called the lymphoid system) is part of the immune system . The system moves lymph, a clear fluid containing white blood cells , through your bloodstream.  

The lymphatic system involves many organs, including the tonsils, adenoids, spleen, and thymus. Lymph nodes filter out bacteria and cancer cells and create white blood cells to fight infection. These nodes are found throughout the body (neck, armpits, groin, chest, and abdomen).

This article explains the function of the lymphatic system, lymphatic diseases, and how to improve lymphatic health. 

The lymphatic system includes tissues, vessels, and organs that move fluid throughout the body and fight infection. When excess plasma (the liquid portion of blood) collects in your body’s tissues, the lymphatic system collects it and moves it back into your bloodstream.

Plasma is the liquid component of blood. It makes up 55% of your blood. Red and white blood cells and platelets, suspended in the plasma, make up the remaining portion.

Fighting Infection Starts With Lymph

Lymph is made up of more than plasma. The excess fluid that drains from your cells and tissues is made of many substances, including:

• Damaged cells
• Cancer cells
• Nutrients  

From Your Tissues to Your Lymph Nodes

Lymphatic vessels are tiny blood vessels and tubes that move lymph from tissues to lymph nodes, which filter out damaged and cancer-causing cells. In addition, the lymph nodes create immune cells to fight infection and other harmful substances it detects in the fluid.

You have about 600 lymph nodes in your body. Lymphatic vessels connect this network of lymph nodes.

Through Lymphatic Vessels to Collecting Ducts

Once the lymph passes through the lymph nodes, it continues through the lymphatic vessels until it reaches the collecting ducts. There, it empties before returning the filtered lymph to your bloodstream.

In addition to lymph, lymphatic vessels, lymph nodes, and collecting ducts, many organs are involved in the lymphatic system. Primary lymphatic organs produce immune cells called lymphocytes , while secondary lymphatic organs fight off germs and harmful substances. 

Primary lymphatic organs include:

• Bone marrow : This sponge-like tissue is located inside your bones. Here, immune cells grow and multiply. 
• Thymus : This gland is located behind the breastbone. Immune cells, called T-cells , mature in the thymus. 

Secondary lymphatic organs include:

• Spleen : This organ sits in the left upper corner of the abdomen. The spleen stores immune cells and platelets and break down red blood cells.
• Tonsils : These are located in the throat. They stop germs from entering the body through the mouth and nose. They also use white blood cells to fight infection.
• Lymph nodes : These bean-shaped glands are located throughout the body, including the armpits, groin, and neck. They filter lymph and create antibodies.
• Mucous membranes : These are located through your respiratory and digestive systems. The membranes in your bowel wall, appendix, respiratory and urinary tracts, and vagina fight off foreign substances and infections.

When the lymphatic system malfunctions, some diseases or disorders can occur. These complications can range from infections to blockages to cancer.

A blockage in the lymphatic system often leads to fluid buildup. In addition, blockages can result from scar tissue from surgery, injury, or infection. Examples of disorders associated with blockage include:

• Lymphedema : This is an accumulation of lymph fluid in the body. It most commonly occurs in the arms and legs. It ranges from mild to very painful. It is common in people who have received cancer treatment .
• Lymphatic filariasis : Also called elephantiasis , this infection is caused by a parasite that infects the body through a mosquito bite. The worms invade the lymphatic system, resulting in a blocked lymphatic system and swelling, pain, and disfigurement. 

Inflammation and Infection

Infection and inflammation can lead to problems with the lymphatic system. These include:

• Lymphadenopathy : This is the medical term for swollen lymph nodes. Lymph nodes, themselves, can become infected, but swollen lymph nodes also indicate an infection within the body. Some infections that commonly result in enlarged lymph nodes include mononucleosis , strep throat , HIV , and skin infections . Lymphoma (cancer of the lymph nodes) and leukemia may also be associated with lymphadenopathy.
• Lymphadenitis : This term refers to an infection of the lymph node(s). This infection often occurs due to germs that spread through the lymphatic system from one part of the body to the lymph nodes.
• Lymphangitis : Inflammation of the lymph vessels, which may result from some bacterial infections.
• Lymphocytosis : This condition is a higher than normal amount of lymphocytes, often a result of your body dealing with infection or inflammation. 
• Castleman disease : This condition is an overgrowth of cells in the lymphatic system. An infection causes it. It may result in a full feeling in the abdomen, lumps in the armpits, groin, neck, and weight loss.
• Mesenteric lymphadenitis : This is inflammation of lymph nodes in the abdomen. An infection causes it, and it usually affects children and teenagers. 
• Tonsillitis : An infection of the tonsils, resulting in a very sore throat. 

Congenital or Genetic Conditions

Sometimes malfunctions of the lymphatic system occur when you are born. These conditions include:

• Intestinal lymphangiectasia : This occurs when there is a loss of lymph tissue in the intestines. Most often, children are diagnosed before age 3.
• Lymphangioma : This is a malformation in the vascular lymphatic system, includes cystic hygroma . 
• Lymphangioleiomyomatosis : This is a rare lung disease where cells grow out of control in the lymph nodes, lungs, and kidneys. 
• Autoimmune lymphoproliferative syndrome: This genetic disorder of the lymph nodes, liver, and spleen results in high lymphocytes.

Cancer of the lymphatic system is called lymphoma . It occurs when lymphocytes multiply in the body uncontrollably. There are two categories of lymphoma: Hodgkin lymphoma and non-Hodgkin lymphoma . 

Non-Hodgkin lymphoma is one of the most common cancers in the U.S. It makes up about 4% of all cancers.

The lymphatic system directly impacts other systems of the body. These include:

• Circulatory system : This system, also known as the cardiovascular system, includes the heart, arteries, capillaries, and veins. Cells, fluids, minerals, nutrients, and wastes move between the cardiovascular system and the lymphatic system.
• Immune system : The lymphatic system is a part of the immune system. The immune system’s role is to protect the body from disease and other harmful substances. It is continually working to build up a defense against germs. As a part of the immune system, the lymphatic system plays an important role. As it moves lymph throughout your body, it identifies, filters out, and reacts to foreign substances by creating antibodies.

Your exposure to certain things can impact the lymphatic system. Therefore, there are some ways that you can support a healthy lymphatic system, including:

• Avoiding chemicals like pesticides and harsh cleaning products
• Staying properly hydrated
• Eating nourishing foods
• Getting adequate physical exercise

The lymphatic system is passive. It doesn't have the heart pumping fluid through it or valves (such as in the veins) to prevent the backflow of fluid. It relies on muscle contractions, movement, massage, and gravity (such as elevating the arms or legs) to aid in lymphatic drainage.

When to See a Healthcare Provider

If you notice unusual swelling or extreme fatigue, it's a good idea to contact your healthcare provider to have them evaluate your situation.

The lymphatic system is part of the immune and circulatory systems. It is responsible for moving excess fluid, called lymph, out of tissues and back through the body. During this process, the lymph travels through the lymphatic vessels, lymph nodes, and collecting ducts before cycling back through the body.

Drinking lots of water, eating nourishing food, and getting enough movement can help your lymphatic system flow as it should. Talk to your healthcare provider if you notice signs that your lymphatic system is not functioning optimally.

The organs of the lymphatic system include bone marrow, thymus, spleen, tonsils, lymph nodes, and mucous membranes.

A lymphatic system "detox" involves doing certain things to help your lymphatic system drain more effectively. However, there is not a lot of evidence to support the practice. Some simple, healthy things to try include staying well-hydrated, eating nourishing food, and getting adequate exercise.

If your lymphatic system is blocked, the fluid buildup may result in swelling in your soft tissues. Often this occurs in the arms and legs. It most often occurs in people who have undergone cancer treatment.

MedlinePlus. Lymph system .

Lymphangiomatosis & Gorham’s Disease Alliance. The lymphatic system .

The American Red Cross. The importance of plasma in blood .

Liao S, von der Weid PY. Lymphatic system: an active pathway for immune protection .  Semin Cell Dev Biol . 2015;38:83-89.

Johns Hopkins Medicine. Lymphadenitis .

Institute for Quality and Efficiency in Health Care (IQWiG). What are the organs of the immune system?

MedlinePlus. Lymphangitis .

Merck Manual. Intestinal lymphangiectasia .

American Cancer Society. Key statistics for non-Hodgkin lymphoma .

Aspelund A, Robciuc MR, Karaman S, Makinen T, Alitalo K. Lymphatic system in cardiovascular medicine . Circ Res. 2016;118(3):515-530. doi:10.1161/CIRCRESAHA.115.306544

MedlinePlus. Immune system and disorders . 

By Kathi Valeii Valeii is a Michigan-based freelance writer with a bachelor's degree in communication from Purdue Global.

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The Lymphatic System

Original Author(s): Jess Speller Last updated: December 17, 2018 Revisions: 31

• 1 Lymph Organs
• 2 Lymph Nodes
• 3 Lymph Vessels
• 4 Lymph Fluid
• 5 Clinical Relevance – Lymphoma

The lymphatic system is a series of vessels and nodes that collect and filter excess tissue fluid (lymph), before returning it to the venous circulation. It forms a vital part of the body’s immune defence.

In this article, we shall look at the components of the lymphatic system, their structure and their clinical correlations.

Fig 1 – Overview of the lymphatic system. It contains lymphoid organs, vessels, nodes and lymph fluid.

Lymph Organs

There are a number of organs that contain lymphatic tissue. They are involved in blood filtering and the maturation of lymphocytes.

• Spleen –  Functions mainly as a blood filter, removing old red blood cells. It also plays a role in the immune response.
• Thymus – Responsible for the development and maturation of T lymphocyte cells.
• Red bone marrow – Responsible for maturation of immature lymphocytes, much like the thymus.

In addition, some lymphatic tissue is located in the tonsils, appendix, and in the walls of the gastrointestinal tract.

Lymph Nodes

Lymph nodes are kidney shaped structures which act to filter foreign particles from the blood, and play an important role in the immune response to infection. On average, an adult has around 400 to 450 different lymph nodes spread throughout the body – with the majority located within the abdomen.

Each node contains T lymphocytes, B lymphocytes, and other immune cells. They are exposed to the fluid as it passes through the node, and can mount an immune response if they detect the presence of a pathogen. This immune response often recruits more inflammatory cells into the node – which is why lymph nodes are palpable during infection.

Lymph fluid enters the node through afferent lymphatic channels  and leaves the node via  efferent channels . Macrophages located within the sinuses of the lymph node act to filter foreign particles out of the fluid as it travels through.

Fig 2 – Structure of a lymph node.

Lymph Vessels

The lymphatic vessels transport lymph fluid around the body. There are two main systems of lymph vessels – superficial and deep:

• Superficial vessels – arise in the subcutaneous tissue, and tends to accompany venous flow. They eventually drain into deep vessels.
• Deep vessels – drain the deeper structures of the body, such as the internal organs. They tend to accompany deep arteries.

The drainage of lymph begins in  lymph channels, which start as blind ended capillaries and gradually develop into vessels . These vessels travel proximally, draining through several  lymph nodes .

Eventually the vessels empty into lymphatic trunks  (also known as collecting vessels) – and these eventually converge to form the right lymphatic duct and the thoracic duct.

The right lymphatic duct is responsible for draining the lymph from the upper right quadrant of the body. This includes the right side of the head and neck, the right side of the thorax and the right upper limb. The thoracic duct is much larger and drains lymph from the rest of the body. These two ducts then empty into the venous circulation at the subclavian veins, via the right and left venous angles .

Fig 3 – The left and right lymphatic ducts.

Lymph Fluid

Lymph is a transudative fluid that is transparent and yellow. It is formed when fluid leaves the capillary bed in tissues due to hydrostatic pressure. Roughly 10% of blood volume becomes lymph.

The composition of lymph is fairly similar to that of blood plasma, with the majority of the volume (around 95%) comprised of water. The remaining  5%  is composed of proteins, lipids, carbohydrates (mainly glucose), various ions and some cells (mainly lymphocytes), although this can vary depending on where in the body the lymph is produced. For example, chyle (lymph that is produced in the gastrointestinal system) is particularly rich in fats.

The average adult produces between 3-4 litres of lymphatic fluid each day, although this can vary in illness.

Clinical Relevance – Lymphoma

A lymphoma is one of a group of tumours developing from lymphatic cells. They make up around 3-4% of all cancers worldwide and typically have a 5-year survival rate of 70-85%, depending on the subtype. The two main subtypes are Hodgkin lymphoma  (HL) and non-Hodgkin lymphoma (NHL), with roughly 90% of lymphomas being NHLs. Risk factors for these lymphomas include:

• Infection with Epstein-Barr virus (HL)
• Autoimmune diseases (NHL)
• HIV/AIDs (NHL)
• Eating a large amount of meat and fat (NHL)

Fig 4 – A follicular lymphoma.

A diagnosis is reached following a lymph node biopsy, if histological features of lymphoma are found, further tests such as immunophenotyping can be carried out to determine the subtype.

Symptoms of lymphoma often include:

• Lymphadenopathy – swelling of lymph nodes
• Night sweats
• Weight loss
• Loss of appetite
• Spleen -  Functions mainly as a blood filter, removing old red blood cells. It also plays a role in the immune response.

Lymph nodes are kidney shaped structures which act to filter foreign particles from the blood, and play an important role in the immune response to infection. On average, an adult has around 400 to 450 different lymph nodes spread throughout the body - with the majority located within the abdomen.

Each node contains T lymphocytes, B lymphocytes, and other immune cells. They are exposed to the fluid as it passes through the node, and can mount an immune response if they detect the presence of a pathogen. This immune response often recruits more inflammatory cells into the node - which is why lymph nodes are palpable during infection.

The lymphatic vessels transport lymph fluid around the body. There are two main systems of lymph vessels - superficial and deep:

• Superficial vessels - arise in the subcutaneous tissue, and tends to accompany venous flow. They eventually drain into deep vessels.
• Deep vessels - drain the deeper structures of the body, such as the internal organs. They tend to accompany deep arteries.

Eventually the vessels empty into lymphatic trunks  (also known as collecting vessels) - and these eventually converge to form the right lymphatic duct and the thoracic duct.

The average adult produces between 3-4 litres of lymphatic fluid each day, although this can vary in illness. [start-clinical]

Clinical Relevance - Lymphoma

• Lymphadenopathy - swelling of lymph nodes

[end-clinical]

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7.2: Anatomy of the Lymphatic and Immune Systems

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• Page ID 34614

Learning Objectives

By the end of this section, you will be able to:

• Describe the structure and function of the lymphatic tissue (lymph fluid, vessels, ducts, and organs)
• Describe the structure and function of the primary and secondary lymphatic organs
• Discuss the cells of the immune system, how they function, and their relationship with the lymphatic system

The immune system is the complex collection of cells and organs that destroys or neutralizes pathogens that would otherwise cause disease or death. The lymphatic system, for most people, is associated with the immune system to such a degree that the two systems are virtually indistinguishable. The lymphatic system is the system of vessels, cells, and organs that carries excess fluids to the bloodstream and filters pathogens from the blood. The swelling of lymph nodes during an infection and the transport of lymphocytes via the lymphatic vessels are but two examples of the many connections between these critical organ systems.

Functions of the Lymphatic System

A major function of the lymphatic system is to drain body fluids and return them to the bloodstream. Blood pressure causes leakage of fluid from the capillaries, resulting in the accumulation of fluid in the interstitial space—that is, spaces between individual cells in the tissues. In humans, 20 liters of plasma is released into the interstitial space of the tissues each day due to capillary filtration. Once this filtrate is out of the bloodstream and in the tissue spaces, it is referred to as interstitial fluid. Of this, 17 liters is reabsorbed directly by the blood vessels. But what happens to the remaining three liters? This is where the lymphatic system comes into play. It drains the excess fluid and empties it back into the bloodstream via a series of vessels, trunks, and ducts. Lymph is the term used to describe interstitial fluid once it has entered the lymphatic system. When the lymphatic system is damaged in some way, such as by being blocked by cancer cells or destroyed by injury, protein-rich interstitial fluid accumulates (sometimes “backs up” from the lymph vessels) in the tissue spaces. This inappropriate accumulation of fluid referred to as lymphedema may lead to serious medical consequences.

As the vertebrate immune system evolved, the network of lymphatic vessels became convenient avenues for transporting the cells of the immune system. Additionally, the transport of dietary lipids and fat-soluble vitamins absorbed in the gut uses this system.

Cells of the immune system not only use lymphatic vessels to make their way from interstitial spaces back into the circulation, but they also use lymph nodes as major staging areas for the development of critical immune responses. A lymph node is one of the small, bean-shaped organs located throughout the lymphatic system.

Practice Questions

Visit this website for an overview of the lymphatic system. What are the three main components of the lymphatic system? [reveal-answer q=”206834″]Show Answer[/reveal-answer] [hidden-answer a=”206834″]The three main components are the lymph vessels, the lymph nodes, and the lymph.[/hidden-answer]

Structure of the Lymphatic System

The lymphatic vessels begin as open-ended capillaries, which feed into larger and larger lymphatic vessels, and eventually empty into the bloodstream by a series of ducts. Along the way, the lymph travels through the lymph nodes, which are commonly found near the groin, armpits, neck, chest, and abdomen. Humans have about 500–600 lymph nodes throughout the body.

A major distinction between the lymphatic and cardiovascular systems in humans is that lymph is not actively pumped by the heart, but is forced through the vessels by the movements of the body, the contraction of skeletal muscles during body movements, and breathing. One-way valves (semi-lunar valves) in lymphatic vessels keep the lymph moving toward the heart. Lymph flows from the lymphatic capillaries, through lymphatic vessels, and then is dumped into the circulatory system via the lymphatic ducts located at the junction of the jugular and subclavian veins in the neck.

Lymphatic Capillaries

Lymphatic capillaries , also called the terminal lymphatics, are vessels where interstitial fluid enters the lymphatic system to become lymph fluid. Located in almost every tissue in the body, these vessels are interlaced among the arterioles and venules of the circulatory system in the soft connective tissues of the body. Exceptions are the central nervous system, bone marrow, bones, teeth, and the cornea of the eye, which do not contain lymph vessels.

Lymphatic capillaries are formed by a one cell-thick layer of endothelial cells and represent the open end of the system, allowing interstitial fluid to flow into them via overlapping cells. When interstitial pressure is low, the endothelial flaps close to prevent “backflow.” As interstitial pressure increases, the spaces between the cells open up, allowing the fluid to enter. Entry of fluid into lymphatic capillaries is also enabled by the collagen filaments that anchor the capillaries to surrounding structures. As interstitial pressure increases, the filaments pull on the endothelial cell flaps, opening up them even further to allow easy entry of fluid.

In the small intestine, lymphatic capillaries called lacteals are critical for the transport of dietary lipids and lipid-soluble vitamins to the bloodstream. In the small intestine, dietary triglycerides combine with other lipids and proteins, and enter the lacteals to form a milky fluid called chyle . The chyle then travels through the lymphatic system, eventually entering the liver and then the bloodstream.

Larger Lymphatic Vessels, Trunks, and Ducts

The lymphatic capillaries empty into larger lymphatic vessels, which are similar to veins in terms of their three-tunic structure and the presence of valves. These one-way valves are located fairly close to one another, and each one causes a bulge in the lymphatic vessel, giving the vessels a beaded appearance.

The superficial and deep lymphatics eventually merge to form larger lymphatic vessels known as lymphatic trunks . On the right side of the body, the right sides of the head, thorax, and right upper limb drain lymph fluid into the right subclavian vein via the right lymphatic duct. On the left side of the body, the remaining portions of the body drain into the larger thoracic duct, which drains into the left subclavian vein. The thoracic duct itself begins just beneath the diaphragm in the cisterna chyli , a sac-like chamber that receives lymph from the lower abdomen, pelvis, and lower limbs by way of the left and right lumbar trunks and the intestinal trunk.

The overall drainage system of the body is asymmetrical. The right lymphatic duct receives lymph from only the upper right side of the body. The lymph from the rest of the body enters the bloodstream through the thoracic duct via all the remaining lymphatic trunks. In general, lymphatic vessels of the subcutaneous tissues of the skin, that is, the superficial lymphatics, follow the same routes as veins, whereas the deep lymphatic vessels of the viscera generally follow the paths of arteries.

The Organization of Immune Function

The immune system is a collection of barriers, cells, and soluble proteins that interact and communicate with each other in extraordinarily complex ways. The modern model of immune function is organized into three phases based on the timing of their effects. The three temporal phases consist of the following:

• Barrier defenses such as the skin and mucous membranes, which act instantaneously to prevent pathogenic invasion into the body tissues
• The rapid but nonspecific innate immune response , which consists of a variety of specialized cells and soluble factors
• The slower but more specific and effective adaptive immune response , which involves many cell types and soluble factors, but is primarily controlled by white blood cells (leukocytes) known as lymphocytes , which help control immune responses

The cells of the blood, including all those involved in the immune response, arise in the bone marrow via various differentiation pathways from hematopoietic stem cells. In contrast with embryonic stem cells, hematopoietic stem cells are present throughout adulthood and allow for the continuous differentiation of blood cells to replace those lost to age or function. These cells can be divided into three classes based on function:

• Phagocytic cells, which ingest pathogens to destroy them
• Lymphocytes, which specifically coordinate the activities of adaptive immunity
• Cells containing cytoplasmic granules, which help mediate immune responses against parasites and intracellular pathogens such as viruses

Lymphocytes

As stated above, lymphocytes are the primary cells of adaptive immune responses (see Table 1 for more details). The two basic types of lymphocytes, B cells and T cells, are identical morphologically with a large central nucleus surrounded by a thin layer of cytoplasm. They are distinguished from each other by their surface protein markers as well as by the molecules they secrete. While B cells mature in red bone marrow and T cells mature in the thymus, they both initially develop from bone marrow. T cells migrate from bone marrow to the thymus gland where they further mature. B cells and T cells are found in many parts of the body, circulating in the bloodstream and lymph, and residing in secondary lymphoid organs, including the spleen and lymph nodes, which will be described later in this section. The human body contains approximately 10 12 lymphocytes.

B cells are immune cells that function primarily by producing antibodies. An antibody is any of the group of proteins that binds specifically to pathogen-associated molecules known as antigens. An antigen is a chemical structure on the surface of a pathogen that binds to T or B lymphocyte antigen receptors. Once activated by binding to antigen, B cells differentiate into cells that secrete a soluble form of their surface antibodies. These activated B cells are known as plasma cells.

The T cell , on the other hand, does not secrete antibody but performs a variety of functions in the adaptive immune response. Different T cell types have the ability to either secrete soluble factors that communicate with other cells of the adaptive immune response or destroy cells infected with intracellular pathogens. The roles of T and B lymphocytes in the adaptive immune response will be discussed further in this chapter.

Plasma Cells

Another type of lymphocyte of importance is the plasma cell. A plasma cell is a B cell that has differentiated in response to antigen binding, and has thereby gained the ability to secrete soluble antibodies. These cells differ in morphology from standard B and T cells in that they contain a large amount of cytoplasm packed with the protein-synthesizing machinery known as rough endoplasmic reticulum.

Natural Killer Cells

A fourth important lymphocyte is the natural killer cell, a participant in the innate immune response. A natural killer cell (NK) is a circulating blood cell that contains cytotoxic (cell-killing) granules in its extensive cytoplasm. It shares this mechanism with the cytotoxic T cells of the adaptive immune response. NK cells are among the body’s first lines of defense against viruses and certain types of cancer.

Practice Question

Visit this website to learn about the many different cell types in the immune system and their very specialized jobs. What is the role of the dendritic cell in an HIV infection? [reveal-answer q=”605750″]Show Answer[/reveal-answer] [hidden-answer a=”605750″]The dendritic cell transports the virus to a lymph node.[/hidden-answer]

Primary Lymphoid Organs and Lymphocyte Development

Understanding the differentiation and development of B and T cells is critical to the understanding of the adaptive immune response. It is through this process that the body (ideally) learns to destroy only pathogens and leaves the body’s own cells relatively intact. The primary lymphoid organs are the bone marrow, spleen, and thymus gland. The lymphoid organs are where lymphocytes mature, proliferate, and are selected, which enables them to attack pathogens without harming the cells of the body.

Bone Marrow

In the embryo, blood cells are made in the yolk sac. As development proceeds, this function is taken over by the spleen, lymph nodes, and liver. Later, the bone marrow takes over most hematopoietic functions, although the final stages of the differentiation of some cells may take place in other organs. The red bone marrow is a loose collection of cells where hematopoiesis occurs, and the yellow bone marrow is a site of energy storage, which consists largely of fat cells. The B cell undergoes nearly all of its development in the red bone marrow, whereas the immature T cell, called a thymocyte , leaves the bone marrow and matures largely in the thymus gland.

The thymus gland is a bilobed organ found in the space between the sternum and the aorta of the heart. Connective tissue holds the lobes closely together but also separates them and forms a capsule.

The connective tissue capsule further divides the thymus into lobules via extensions called trabeculae. The outer region of the organ is known as the cortex and contains large numbers of thymocytes with some epithelial cells, macrophages, and dendritic cells (two types of phagocytic cells that are derived from monocytes). The cortex is densely packed so it stains more intensely than the rest of the thymus. The medulla, where thymocytes migrate before leaving the thymus, contains a less dense collection of thymocytes, epithelial cells, and dendritic cells.

Aging and the Immune System

By the year 2050, 25 percent of the population of the United States will be 60 years of age or older. The CDC estimates that 80 percent of those 60 years and older have one or more chronic disease associated with deficiencies of the immune systems. This loss of immune function with age is called immunosenescence. To treat this growing population, medical professionals must better understand the aging process. One major cause of age-related immune deficiencies is thymic involution, the shrinking of the thymus gland that begins at birth, at a rate of about three percent tissue loss per year, and continues until 35–45 years of age, when the rate declines to about one percent loss per year for the rest of one’s life. At that pace, the total loss of thymic epithelial tissue and thymocytes would occur at about 120 years of age. Thus, this age is a theoretical limit to a healthy human lifespan.

Thymic involution has been observed in all vertebrate species that have a thymus gland. Animal studies have shown that transplanted thymic grafts between inbred strains of mice involuted according to the age of the donor and not of the recipient, implying the process is genetically programmed. There is evidence that the thymic microenvironment, so vital to the development of naïve T cells, loses thymic epithelial cells according to the decreasing expression of the FOXN1 gene with age.

It is also known that thymic involution can be altered by hormone levels. Sex hormones such as estrogen and testosterone enhance involution, and the hormonal changes in pregnant women cause a temporary thymic involution that reverses itself, when the size of the thymus and its hormone levels return to normal, usually after lactation ceases. What does all this tell us? Can we reverse immunosenescence, or at least slow it down? The potential is there for using thymic transplants from younger donors to keep thymic output of naïve T cells high. Gene therapies that target gene expression are also seen as future possibilities. The more we learn through immunosenescence research, the more opportunities there will be to develop therapies, even though these therapies will likely take decades to develop. The ultimate goal is for everyone to live and be healthy longer, but there may be limits to immortality imposed by our genes and hormones.

Secondary Lymphoid Organs and their Roles in Active Immune Responses

Lymphocytes develop and mature in the primary lymphoid organs, but they mount immune responses from the secondary lymphoid organs . A naïve lymphocyte is one that has left the primary organ and entered a secondary lymphoid organ. Naïve lymphocytes are fully functional immunologically, but have yet to encounter an antigen to respond to. In addition to circulating in the blood and lymph, lymphocytes concentrate in secondary lymphoid organs, which include the lymph nodes, spleen, and lymphoid nodules. All of these tissues have many features in common, including the following:

• The presence of lymphoid follicles, the sites of the formation of lymphocytes, with specific B cell-rich and T cell-rich areas
• An internal structure of reticular fibers with associated fixed macrophages
• Germinal centers , which are the sites of rapidly dividing B lymphocytes and plasma cells, with the exception of the spleen
• Specialized post-capillary vessels known as high endothelial venules ; the cells lining these venules are thicker and more columnar than normal endothelial cells, which allow cells from the blood to directly enter these tissues

Lymph Nodes

Lymph nodes function to remove debris and pathogens from the lymph, and are thus sometimes referred to as the “filters of the lymph”. Any bacteria that infect the interstitial fluid are taken up by the lymphatic capillaries and transported to a regional lymph node. Dendritic cells and macrophages within this organ internalize and kill many of the pathogens that pass through, thereby removing them from the body. The lymph node is also the site of adaptive immune responses mediated by T cells, B cells, and accessory cells of the adaptive immune system. Like the thymus, the bean-shaped lymph nodes are surrounded by a tough capsule of connective tissue and are separated into compartments by trabeculae, the extensions of the capsule. In addition to the structure provided by the capsule and trabeculae, the structural support of the lymph node is provided by a series of reticular fibers laid down by fibroblasts.

The major routes into the lymph node are via afferent lymphatic vessels . Cells and lymph fluid that leave the lymph node may do so by another set of vessels known as the efferent lymphatic vessels . Lymph enters the lymph node via the subcapsular sinus, which is occupied by dendritic cells, macrophages, and reticular fibers. Within the cortex of the lymph node are lymphoid follicles, which consist of germinal centers of rapidly dividing B cells surrounded by a layer of T cells and other accessory cells. As the lymph continues to flow through the node, it enters the medulla, which consists of medullary cords of B cells and plasma cells, and the medullary sinuses where the lymph collects before leaving the node via the efferent lymphatic vessels.

In addition to the lymph nodes, the spleen is a major secondary lymphoid organ. It is about 12 cm (5 in) long and is attached to the lateral border of the stomach via the gastrosplenic ligament. The spleen is a fragile organ without a strong capsule, and is dark red due to its extensive vascularization. The spleen is sometimes called the “filter of the blood” because of its extensive vascularization and the presence of macrophages and dendritic cells that remove microbes and other materials from the blood, including dying red blood cells. The spleen also functions as the location of immune responses to blood-borne pathogens.

The spleen is also divided by trabeculae of connective tissue, and within each splenic nodule is an area of red pulp, consisting of mostly red blood cells, and white pulp, which resembles the lymphoid follicles of the lymph nodes. Upon entering the spleen, the splenic artery splits into several arterioles (surrounded by white pulp) and eventually into sinusoids. Blood from the capillaries subsequently collects in the venous sinuses and leaves via the splenic vein. The red pulp consists of reticular fibers with fixed macrophages attached, free macrophages, and all of the other cells typical of the blood, including some lymphocytes. The white pulp surrounds a central arteriole and consists of germinal centers of dividing B cells surrounded by T cells and accessory cells, including macrophages and dendritic cells. Thus, the red pulp primarily functions as a filtration system of the blood, using cells of the relatively nonspecific immune response, and white pulp is where adaptive T and B cell responses are mounted.

Lymphoid Nodules

The other lymphoid tissues, the lymphoid nodules , have a simpler architecture than the spleen and lymph nodes in that they consist of a dense cluster of lymphocytes without a surrounding fibrous capsule. These nodules are located in the respiratory and digestive tracts, areas routinely exposed to environmental pathogens.

Tonsils are lymphoid nodules located along the inner surface of the pharynx and are important in developing immunity to oral pathogens. The tonsil located at the back of the throat, the pharyngeal tonsil, is sometimes referred to as the adenoid when swollen. Such swelling is an indication of an active immune response to infection. Histologically, tonsils do not contain a complete capsule, and the epithelial layer invaginates deeply into the interior of the tonsil to form tonsillar crypts. These structures, which accumulate all sorts of materials taken into the body through eating and breathing, actually “encourage” pathogens to penetrate deep into the tonsillar tissues where they are acted upon by numerous lymphoid follicles and eliminated. This seems to be the major function of tonsils—to help children’s bodies recognize, destroy, and develop immunity to common environmental pathogens so that they will be protected in their later lives. Tonsils are often removed in those children who have recurring throat infections, especially those involving the palatine tonsils on either side of the throat, whose swelling may interfere with their breathing and/or swallowing.

Mucosa-associated lymphoid tissue (MALT) consists of an aggregate of lymphoid follicles directly associated with the mucous membrane epithelia. MALT makes up dome-shaped structures found underlying the mucosa of the gastrointestinal tract, breast tissue, lungs, and eyes. Peyer’s patches, a type of MALT in the small intestine, are especially important for immune responses against ingested substances. Peyer’s patches contain specialized endothelial cells called M (or microfold) cells that sample material from the intestinal lumen and transport it to nearby follicles so that adaptive immune responses to potential pathogens can be mounted.

Bronchus-associated lymphoid tissue (BALT) consists of lymphoid follicular structures with an overlying epithelial layer found along the bifurcations of the bronchi, and between bronchi and arteries. They also have the typically less-organized structure of other lymphoid nodules. These tissues, in addition to the tonsils, are effective against inhaled pathogens.

Chapter Review

The lymphatic system is a series of vessels, ducts, and trunks that remove interstitial fluid from the tissues and return it the blood. The lymphatics are also used to transport dietary lipids and cells of the immune system. Cells of the immune system all come from the hematopoietic system of the bone marrow. Primary lymphoid organs, the bone marrow and thymus gland, are the locations where lymphocytes of the adaptive immune system proliferate and mature. Secondary lymphoid organs are site in which mature lymphocytes congregate to mount immune responses. Many immune system cells use the lymphatic and circulatory systems for transport throughout the body to search for and then protect against pathogens.

Answer the question(s) below to see how well you understand the topics covered in the previous section.

https://oea.herokuapp.com/assessments/256

Critical Thinking Question

Describe the flow of lymph from its origins in interstitial fluid to its emptying into the venous bloodstream.

[reveal-answer q=”822756″]Show Answer[/reveal-answer] [hidden-answer a=”822756″]The lymph enters through lymphatic capillaries, and then into larger lymphatic vessels. The lymph can only go in one direction due to valves in the vessels. The larger lymphatics merge to form trunks that enter into the blood via lymphatic ducts. [/hidden-answer]

adaptive immune response: relatively slow but very specific and effective immune response controlled by lymphocytes

afferent lymphatic vessels: lead into a lymph node

antibody: antigen-specific protein secreted by plasma cells; immunoglobulin

antigen: molecule recognized by the receptors of B and T lymphocytes

barrier defenses: antipathogen defenses deriving from a barrier that physically prevents pathogens from entering the body to establish an infection

B cells: lymphocytes that act by differentiating into an antibody-secreting plasma cell

bone marrow: tissue found inside bones; the site of all blood cell differentiation and maturation of B lymphocytes

bronchus-associated lymphoid tissue (BALT): lymphoid nodule associated with the respiratory tract

chyle: lipid-rich lymph inside the lymphatic capillaries of the small intestine

cisterna chyli: bag-like vessel that forms the beginning of the thoracic duct

efferent lymphatic vessels: lead out of a lymph node

germinal centers: clusters of rapidly proliferating B cells found in secondary lymphoid tissues

high endothelial venules: vessels containing unique endothelial cells specialized to allow migration of lymphocytes from the blood to the lymph node

immune system: series of barriers, cells, and soluble mediators that combine to response to infections of the body with pathogenic organisms

innate immune response: rapid but relatively nonspecific immune response

lymph: fluid contained within the lymphatic system

lymph node: one of the bean-shaped organs found associated with the lymphatic vessels

lymphatic capillaries: smallest of the lymphatic vessels and the origin of lymph flow

lymphatic system: network of lymphatic vessels, lymph nodes, and ducts that carries lymph from the tissues and back to the bloodstream.

lymphatic trunks: large lymphatics that collect lymph from smaller lymphatic vessels and empties into the blood via lymphatic ducts

lymphocytes: white blood cells characterized by a large nucleus and small rim of cytoplasm

lymphoid nodules: unencapsulated patches of lymphoid tissue found throughout the body

mucosa-associated lymphoid tissue (MALT): lymphoid nodule associated with the mucosa

naïve lymphocyte: mature B or T cell that has not yet encountered antigen for the first time

natural killer cell (NK): cytotoxic lymphocyte of innate immune response

plasma cell: differentiated B cell that is actively secreting antibody

primary lymphoid organ: site where lymphocytes mature and proliferate; red bone marrow and thymus gland

right lymphatic duct: drains lymph fluid from the upper right side of body into the right subclavian vein

secondary lymphoid organs: sites where lymphocytes mount adaptive immune responses; examples include lymph nodes and spleen

spleen: secondary lymphoid organ that filters pathogens from the blood (white pulp) and removes degenerating or damaged blood cells (red pulp)

T cell: lymphocyte that acts by secreting molecules that regulate the immune system or by causing the destruction of foreign cells, viruses, and cancer cells

thoracic duct: large duct that drains lymph from the lower limbs, left thorax, left upper limb, and the left side of the head

thymocyte: immature T cell found in the thymus

thymus: primary lymphoid organ; where T lymphocytes proliferate and mature

tonsils: lymphoid nodules associated with the nasopharynx

Contributors and Attributions

• Anatomy & Physiology. Provided by : OpenStax CNX. Located at : http://cnx.org/contents/[email protected] . License : CC BY: Attribution . License Terms : Download for free at http://cnx.org/contents/[email protected]

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19.6: Practice Practical- Lymphatic and Immune System

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Lymphatic System

Definition: Lymphatic System is a collection of organs, tissues, ducts, and vessels that help to make or transport lymph. Lymph is a clear-to-white fluid circulating in the vessels and ducts of the lymphatic system that contains white blood cells, proteins, and fat. It was first described in the seventeenth century independently by Olaus Rudbeck and Thomas Bartholin. The human circulatory system processes an average of 20 liters of blood per day through capillary filtration, which removes plasma while leaving the blood cells. Roughly 17 liters of the filtered plasma is reabsorbed directly into the blood vessels, while the remaining three liters remain in the interstitial fluid. One of the main functions of the lymph system is to provide an accessory return route to the blood for the surplus three liters.

There are about 600 lymph nodes in the body. These nodes swell in response to infection, due to a build-up of lymph fluid, bacteria, or other organisms and immune system cells.

A person with a throat infection, for example, may feel that their “glands” are swollen. Swollen glands can be felt especially under the jaw, in the armpits, or in the groin area. These are, in fact, not glands but lymph nodes.

The lymphatic system has three main functions:

• It maintains the balance of fluid between the blood and tissues, known as fluid homeostasis.
• It forms part of the body’s immune system and helps defend against bacteria and other intruders.
• It facilitates absorption of fats and fat-soluble nutrients in the digestive system.

The system has special small vessels called lacteals. These enable it to absorb fats and fat-soluble nutrients from the gut.

Structure and Functions of Lymphatic System: The lymphatic system consists of lymphatic organs, a conducting network of lymphatic vessels, and the circulating lymph.

The primary or central lymphoid organs generate lymphocytes from immature progenitor cells. The thymus and the bone marrow constitute the primary lymphoid organs involved in the production and early clonal selection of lymphocyte tissues.

Secondary or peripheral lymphoid organs, which include lymph nodes and the spleen, maintain mature naive lymphocytes and initiate an adaptive immune response. Secondary lymphoid tissue provides the environment for the foreign or altered native molecules (antigens) to interact with the lymphocytes. It is exemplified by the lymph nodes and the lymphoid follicles in tonsils, Peyer’s patches, spleen, adenoids, skin, etc. that are associated with the mucosa-associated lymphoid tissue (MALT).

• The thymus gland – The thymus gland is a lymphatic organ and an endocrine gland that is found just behind the sternum. It secretes hormones and is crucial in the production, maturation, and differentiation of immune T cells. It is active in developing the immune system from before birth and through childhood.
• Spleen – The spleen is not connected to the lymphatic system in the same way as lymph nodes, but it is lymphoid tissue. This means it plays a role in the production of white blood cells that form part of the immune system. Its other major role is to filter the blood to remove microbes and old and damaged red blood cells and platelets.
• The bone marrow – Bone marrow is not lymphatic tissue, but it can be considered part of the lymphatic system because it is here that the B cell lymphocytes of the immune system mature. This tissue is located inside many of the bones in your body and is a tissue that produces red blood cells, white blood cells, and platelets.
• Lymph nodes – The lymph nodes, or lymph glands, are small, encapsulated bean-shaped structures composed of lymphatic tissue. Thousands of lymph nodes are found throughout the body along the lymphatic routes, and they are especially prevalent in areas around the armpits (axillary nodes), groin (inguinal nodes), neck (cervical nodes), and knees (popliteal nodes). The nodes contain lymphocytes, which enter from the bloodstream via specialized vessels called the high endothelial venules.

The lymph system has three main functions.

Fluid balance: The lymphatic system helps maintain fluid balance. It returns excess fluid and proteins from the tissues that cannot be returned through the blood vessels. The fluid is found in tissue spaces and cavities, in the tiny spaces surrounding cells, known as the interstitial spaces. These are reached by the smallest blood and lymph capillaries.

Around 90 percent of the plasma that reaches tissues from the arterial blood capillaries is returned by the venous capillaries and back along veins. The remaining 10 percent is drained back by the lymphatics.

Absorption: Most of the fats absorbed from the gastrointestinal tract are taken up in a part of the gut membrane in the small intestine that is specially adapted by the lymphatic system. The lymphatic system has tiny lacteals in this part of the intestine that form part of the villi. These finger-like protruding structures are produced by the tiny folds in the absorptive surface of the gut.

Lacteals absorb fats and fat-soluble vitamins to form a milky white fluid called chyle. This fluid contains lymph and emulsified fats, or free fatty acids. It delivers nutrients indirectly when it reaches the venous blood circulation. Blood capillaries take up other nutrients directly.

The immune system: The lymphatic system plays a major role in the body’s immune system, as the primary site for cells relating to an adaptive immune system including T-cells and B-cells. Cells in the lymphatic system react to antigens presented or found by the cells directly or by other dendritic cells. When an antigen is recognized, an immunological cascade begins involving the activation and recruitment of more and more cells, the production of antibodies and cytokines and the recruitment of other immunological cells such as macrophages.

If the immune system is not able to fight off these micro-organisms, or pathogens, they can be harmful and even fatal.

A number of different immune cells and special molecules work together to fight off the unwanted pathogens.

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Lymphatic System: Diagram and Anatomy

Nearly 20 litres of blood plasma are filtered through capillaries daily to bathe and nourish the body’s cells. The lymphatic system processes almost 20% of this fluid, which seeps from capillaries and cells. This tissue fluid travels primarily through the veins and returns to the heart.

Lymphatic System

The lymphatic system is a complex network of tubes, tissues, and organs that is a part of the circulatory system in the vertebrate body.

The lymphatic system removes extra fluid and waste from tissues and deposits them in the bloodstream to support the body’s fluid balance. It also provides the body with disease-fighting lymphocytes, which help in the body’s defence against infection.

The lymphatic system comprises lymph nodes, open-ended vessels, and organs like the tonsils, spleen, and thymus. The lymphatic system causes the lymph to move only in the direction of heart.

Diagram of Lymphatic System

The best defence mechanism in the human body is the immune system. It protects us from harmful microorganisms and keeps us healthy.

Anatomy of Lymphatic System

• The extra fluid that leaks from cells and tissues is gathered together in lymph, also known as lymphatic fluid. White blood cells (lymphocytes) that the lymph transports combat infections.
• The bean-shaped glands called lymph nodes regulate and cleanse the lymph as it filters through them. They remove the cancerous and damaged cells by filtration.
• The thymus gland, a lymphatic and endocrine gland, is located behind the sternum. It secretes hormones and is essential for immunological T cell development, maturation, and differentiation.
• The tonsils generate lymphocytes and antibodies. They can provide defence against ingested and inhaled foreign particles.
• Although it is lymphoid tissue, the spleen is not a component of the connected lymphatic system. It produces white blood cells and filters blood to eliminate pathogens, damaged red blood cells, and platelets.
• Although bone marrow is not a lymphatic tissue, it is a part of the lymphatic system because this is where the immune system’s B cell cells develop.

Visit BYJU’S for more such diagrammatic representations related to NEET .

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Immune and Lymphatic System

Welcome to our Immune and Lymphatic System lesson plan! This plan is designed to help students understand the function of these systems within the body. Although we study human systems as independent systems, they are all interdependent to function correctly.

By the end of this lesson, students will be able to explain the role of the immune and lymphatic systems. They will see how they work together to keep our bodies healthy. The Immune and Lymphatic System lesson plan like this one are essential for a well-rounded education in human physiology.

Description

Additional information, what our immune and lymphatic system lesson plan includes.

Lesson Objectives and Overview: Immune and Lymphatic System explains the functions of these important body systems. Students will learn how our bodies fight bad germs to help keep us healthy. They will also understand how the immune system works with other body systems to ensure we can function properly. This lesson is for students in 4th grade, 5th grade, and 6th grade.

Classroom Procedure

Every lesson plan provides you with a classroom procedure page that outlines a step-by-step guide to follow. You do not have to follow the guide exactly. The guide helps you organize the lesson and details when to hand out worksheets. It also lists information in the yellow box that you might find useful. You will find the lesson objectives, state standards, and number of class sessions the lesson should take to complete in this area. In addition, it describes the supplies you will need as well as what and how you need to prepare beforehand.

Options for Lesson

You will find several additional ideas and activities to incorporate into the lesson in the “Options for Lesson” section of the classroom procedure page. One idea is to expand this lesson to talk about vaccines as well. Given the recent Covid-19 vaccine information, you could also discuss the history and use of vaccines. Another option is to invite a doctor to talk to the class and answer questions. The doctor could discuss keeping the immune system healthy, why it is is essential, and what the consequences are of having a non-working or low-functioning immune system.

Teacher Notes

The paragraph on this page gives you a little more information on the lesson overall and describes what you may want to focus your teaching on. You could benefit from teaching this lesson in conjunction with others that deal with germs, vaccines, and so on. The blank lines are available for you to write out any thoughts or ideas you have as you prepare.

IMMUNE AND LYMPHATIC SYSTEM LESSON PLAN CONTENT PAGES

What is the immune system.

The Immune and Lymphatic System contains three pages of content. Students will learn that to be immune means to be protected from disease or illness. Our bodies need a way to defend themselves against invaders, and the immune system is our defense. An immune system is a group of cells, tissues, and organs that protect us from pathogens and keep us healthy. So when we need to fight off a sickness, our immune system goes into battle!

The immune system battles pathogens, allergens, toxins, and malignant cells. Pathogens are microorganisms that can produce diseases. Allergens are substances that cause allergic reactions. Toxins are poisonous, harmful substances, and malignant cells are the cells that can cause cancer. The immune system comprises lots of things, including white blood cells, bone marrow, the spleen, and the lymphatic system. This lesson goes into the role that each part plays.

Parts of the Immune System

Our bone marrow is actually responsible for producing white blood cells. White blood cells include lymphocytes as well, such as B-cells, T-cells, and natural killer cells. As part of the lymphatic system, they are the major player in our immune system. White blood cells move through blood and tissue, seeking any invading microbes. These microbes could be bacteria, viruses, parasites, or even fungi. As soon as they spot an invader, they launch their attack.

The body needs help fighting microbes and the toxins they produce. On the surface of microbes are antigens. Antibodies in our immune system can recognize toxic or foreign antigens. When they do, the microbe is marked for destruction. Cells, proteins, and other chemicals rush to the marked microbe and attack. In addition, a spongy tissue called bone marrow lives inside our bones. It makes red blood cells that help the body carry oxygen, white blood cells to help fight off infections, and platelets that help the blood clot.

Something called the thymus monitors our body’s blood and filters it. It also produces a special type of white blood cell called T-lymphocytes. Another important part is the spleen. A spleen helps remove microbes by filtering the blood and destroys old or damaged red blood cells. Most importantly, it makes antibodies and lymphocytes to support our immune system as it fights off any microbe invaders.

The complement system is a big word for the proteins that rush to help the antibodies attack marked microbes. Finally, there’s the lymphatic system. Inside our body is a network of delicate tubes. These tubes make up the lymphatic system and are part of the immune system. These tubes carry a colorless liquid called lymph. The lymphatic system is made up of lymph nodes, lymph vessels, and lymphocytes.

Lymphatic System

The lymphatic system deals with bacteria, cancer cells, and other cells or cell toxins that could cause disease. It also works to help manage the fluid levels in the body. For example, it drains the extra water, protein, and other substances that leak out of tiny blood capillaries. It also absorbs some of the fats we eat from the intestine. If the lymphatic system didn’t drain excess fluid in our bodies, it would collect and instead cause us to swell.

The immune system does a fantastic job of keeping our bodies healthy and free from invaders. However, every now and then, a microbe slips by, which can cause our bodies to get sick. When this happens, the immune system kicks it up a notch and fights even harder for us to get well.

There are many things we can do to help keep our immune system healthy. Washing our hands, eating nutritious food, exercising, and getting a good night’s sleep are great examples. In addition, we should avoid prolonged exposure to toxic chemicals like pesticides or cleaning fluids. Drinking lots of water is another way to keep our immune system functioning correctly.

IMMUNE AND LYMPHATIC SYSTEM LESSON PLAN WORKSHEETS

The Immune and Lymphatic System lesson plan includes three worksheets: an activity worksheet, a practice worksheet, and a homework assignment. Each one will help students solidify their grasp of the material they learned throughout the lesson. You can refer to the classroom procedure guidelines to know when to hand out each worksheet.

HELP WANTED AD ACTIVITY WORKSHEET

For the activity, students will create ads to recruit white blood cells. They will first read the passage at the top of the worksheet to get background information for the ad. Then they will create a “Help Wanted” flyer that includes four specific parts. First, they must include a job description for what the cells will need to do. They must also include the qualifications or skills required for the job. The third part to include is the benefits, which are what the white blood cells would get for doing their job. Finally, they will include the hours that the cells must work.

IMMUNE AND LYMPHATIC SYSTEM PRACTICE WORKSHEET

The practice worksheet requires students to research four organisms—viruses, bacteria, parasites, and fungi. Using the internet or other sources, they will find information to write a description for each organism. They will also provide three examples, and they will draw an example of each organism in the space provided.

GERM ARMY GENERAL HOMEWORK ASSIGNMENT

Students will read the passage at the top of the page that provides a scenario in which they are the general of a germ army. Their mission is to invade a person’s body and make them sick. However, the person’s immune system is fighting back, so the general must call “Germ Headquarters” and report what’s going on.

The students must write a dialogue between themselves and headquarters about the mission. They should describe what’s happening with the army and what’s happening inside the person’s body.

Worksheet Answer Keys

The lesson plan provides an answer key for the practice worksheet. The correct answers are in red to make it easy to compare them with students’ responses. Given the nature of the assignment, there may be some variation in students’ answers. Keep that in mind as you grade. If you choose to administer the lesson pages to your students via PDF, you will need to save a new file that omits these pages. Otherwise, you can simply print out the applicable pages and keep these as reference for yourself when grading assignments.

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