What makes an organ a lymphoid organ

The surviving cells will not mistake self-molecules for antigens. The thymus is most active early in life for building a large reservoir of T cells. Though removal of the thymus in childhood causes severe immunodeficiency, later in life this is not an issue because of the proliferation of thymus activity early in life.

Central tolerance is another function of the thymus. Autoimmune diseases occur when central tolerance is lost, which causes lymphocytes to recognize host molecules as antigens and attack them, even if those tissues otherwise function normally. The thymus sorts T cells so that they will be inactive towards host molecules, though sometimes a few T cells evade this sorting process and may initiate an autoimmune disease. The spleen, similar to a large lymph node, acts primarily as a blood filter in the mononuclear phagocyte system of the immune system.

Survival is possible without a spleen. However, retrospective epidemiological studies of World War II veterans found that those who had their spleens removed on the battlefield showed significant mortality risk from pneumonia and a significant excess of mortality from ischemic heart disease, but not from other conditions. The spleen is the largest distinct organ of the lymphatic system.

Similar in structure to a large lymph node, it acts primarily as a blood filter. Despite this important function, a healthy life is possible after removal. The spleen plays important roles in regards to red blood cells and the immune system. The spleen is located in the left upper quadrant of the abdomen. It is similar to an enlarged lymph node but is a bit more complex.

The spleen is made up of two distinct tissue types:. Unlike lymph nodes, the spleen possesses only efferent lymphatic vessels, because it only filters blood instead of lymph fluid.

The splenic artery forms its primary blood supply. The spleen is unique in respect to its development within the gut because it is derived from mesenchymal tissue rather than endoderm tissue during embryonic development. However, it still shares the same blood supply as the foregut organs in the abdominal cavity. Spleen : This diagram of the spleen indicates the vein, artery, white pulp, red pulp, and capsule.

The primary function of the spleen is blood filtration. Blood cells have a lifespan of roughly days. When blood passes through the red pulp of the spleen, healthy blood cells easily pass, while older red blood cells are caught phagocytized by the macrophages within.

The macrophages also remove pathogens, denatured hemoglobin, and other cellular debris. Iron from old or damaged hemoglobin content in the blood is filtered out and sent to the liver so that new red blood cells can be created. Antigens are also filtered by the red pulp, which may be presented to naive lymphocytes in the white pulp of the spleen. This stimulates the same type of adaptive immune response that occurs in the lymph nodes.

The spleen is also important for generating new red blood cells early in embryonic development, but this function stops after birth.

The spleen may also function as a reservoir of blood and platelets during hypovolemic shock, which occurs when overall tissue perfusion falls due to severe dehydration or severe bleeding or hemorrhage.

During hypovolemic shock, the spleen can release up to a cup of extra blood to help mitigate the complications of fluid loss. The spleen is often removed surgically if it becomes damaged or infected. This causes modest increases in circulating white blood cells and platelets, diminished responsiveness to some vaccines, and increased susceptibility to infection by bacteria and protozoa.

In particular, there is an increased risk to infection from gram negative bacteria that cause pneumonia. Besides these increased risks, the loss of the spleen does not cause major immune system impairment, and most people will still live normal and healthy lives because the lymph nodes and liver perform the same functions as the spleen.

In particular, those with splenomegaly an enlarged spleen that could rupture or splenic cancers are typically better off living without their spleen than living with the risk of severe bleeding from a ruptured spleen or the plethora of symptoms caused by splenic cancer and its metastases.

The tonsils are small masses of secondary lymphoid tissue located in the pharynx. They function similarly to other types of secondary lymphoid organs and also capture antigens from respiratory tract pathogens. There are four pairs of tonsils located within the pharynx. More information about lymph nodes.

Tissue fluid is filtered by non-encapsulated or partially encapsulated aggregations of lymphoid tissue sometimes called Mucosa Associated Lymphoid Tissue MALT. They are usually small, around 1mm in diameter, with the exceptions beting the tonsils , peyers patches and the appendix. These lymphoid aggregations are frequently found close to moist epithelial surfaces e.

Although the epithelia of these tissues has mechanisms to keep bacteria etc out of the body, this is not foolproof. Thus the lymphoid cells in these areas are able to respond to any bacteria or micro-organisms that do get through the epithelia. Activated B-cells in these areas can develop into plasma cells, and produce antibodies, in situ. Lymphocytes from the larger permanent organs such as the tonsils are able to patrol the surrounding tissue, and quickly respond to foreign antigens.

Tonsils are large partially-encapsulated masses of lymphoid tissue, found in the walls of the pharynx and nasopharynx, and at the base of the tongue. They form an incomplete ring around the gastrointestinal and respiratory tracts, where they cross over. Learn more about tonsils. Peyer's patches are large masses of confluent lymphoid follicles, found in the walls of the ileum, part of the small intestine.

The appendix is covered in the topic Digestive System, Appendix. Lymph nodes are crucial to most antibody-mediated immune responses: when the phagocytic macrophages trap pathogenic material, that material is presented to the lymphocytes so antibodies can be generated. Each lymph node is supplied by one or more afferent lymphatic vessels, which deliver crude, unmodified lymph directly from neighbouring tissues. A healthy, fully functioning node removes the majority of pathogens from the lymph before the fluid leaves via one or more efferent lymphatic vessels.

In addition to its lymphatic supply, each lymph node is supplied with blood via a small artery; the artery delivers a variety of leucocytes, which populate the inner regions of the node. When infection is present, the lymph nodes become increasingly metabolically active and their oxygen requirements increase. A small vein carries deoxygenated blood away from each node and returns it to the major veins. In times of infection, this venous blood may carry a variety of chemical messengers cytokines that are produced by the resident leucocytes in the nodes.

These cytokines act as general warning signals, alerting the body to the potential threat and activating a variety of specific immune reactions. The structure of a lymph node is not unlike that of the spleen. Each lymph node is divided into several regions:. During infection, antibody-producing B-cells begin to proliferate in the germinal centres, causing the affected lymph nodes to enlarge and become palpable and tender.

Some of the cytokines released are pyrogenic meaning they cause fever and act directly on the thermoregulatory centre in the hypothalamus to increase body temperature.

Swollen lymph nodes and a fever are both sure signs that the body is mounting an effective immune response against the offending pathogen; this will be discussed in more detail in part 3 of this series. Other types of lymphatic tissue also exist.

Mucosa-associated lymphoid tissue MALT is positioned to protect the respiratory and gastrointestinal tracts from invasion by microbes. The following are made up of MALT:. The tonsils are aggregates of lymphatic tissue strategically located to prevent foreign material and pathogens from entering the body. The palatine tonsils are in the pharynx, the lingual tonsils in the oral cavity and the pharyngeal tonsils adenoids are at the back of the nasal cavity; as a result of this, the tonsils themselves are at high risk of infection and inflammation tonsillitis.

This will also be discussed further in part 3. Sign in or Register a new account to join the discussion. You are here: Immunology. The lymphatic system 2: structure and function of the lymphoid organs. Abstract This article is the second in a six-part series about the lymphatic system. This article has been double-blind peer reviewed Scroll down to read the article or download a print-friendly PDF here if the PDF fails to fully download please try again using a different browser Click here to see other articles in this series Assess your knowledge and gain CPD evidence by taking the Nursing Times Self-assessment test.

Key points The lymphoid organs include the red bone marrow, thymus, spleen and clusters of lymph nodes Blood and immune cells are produced inside the red bone marrow, during a process called haematopoiesis The thymus secretes hormones that are essential for normal immune function and develops T-lymphocytes The spleen mounts the immune response and removes micro-organisms and damaged red blood cells from circulation Lymph nodes are clustered throughout the body and filter pathogens from lymph, swelling when mounting an immune response.

Also in this series The lymphatic system 1: structure, function and oedema The lymphatic system 3: its role in the immune system The lymphatic system 4: allergies, anaphylaxis and anaphylactic shock The lymphatic system 5: vaccinations and immunological memory The lymphatic system 6: the history and function of immunotherapies. References Armstrong RA et al Successful non-operative management of haemodynamically unstable traumatic splenic injuries: 4-year case series in a UK major trauma centre.

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Physiological Reports ; 8: 2, e Gujar S et al A cadaveric study of human spleen and its clinical significance. National Journal of Clinical Anatomy ; 6: 1, Polish Journal of Radiology ; 4, Pivkin IV et al Biomechanics of red blood cells in human spleen and consequences for physiology and disease. Severa M et al Thymosins in multiple sclerosis and its experimental models: moving from basic to clinical application. Multiple Sclerosis and Related Disorders ; Sherer Y et al The dual relationship between thymectomy and autoimmunity: the kaleidoscope of autoimmune disease.

In: Paul S ed Autoimmune Reactions. Contemporary Immunology. Totowa, NJ: Humana Press. Sports Medicine ; 6, Related files.