Organs of the immune system

The lymphoid organs play various roles in generating, housing, developing, and deploying lymphocytes. The bone marrow is the source of all blood cells. Lymphocyte precursors from the bone marrow mature into T-cells in the thymus, a small organ located in the upper chest under the breastbone.

A hormone produced by the thymus called thymosin stimulates differentiation and maturation of the lymphoid precursors into either helper, cytotoxic, or regulatory (sometimes called suppressor) T-cells. Mature T-cells leave the thymus and become part of what is called the adaptive immune system, made up of highly specialised cells that prevent, impede, or eliminate pathogenic threats. The thymus shrinks in size over the course of a lifetime and is most active in childhood.

The spleen is located on the left side of the body between the stomach and the diaphragm; it is roughly the size of a fist. It is similar to the lymph nodes in function in that it also houses lymphocytes and macrophages that can filter the blood and eliminate old and damaged cells. Other lobules of the spleen contain red blood cells as well. The spleen stores excess blood and keeps body fluids in balance. One can live without a spleen, but doing so reduces the lymphatic system's filtering capacity, increasing susceptibility to infection.

Non-encapsulated clusters of lymphoid tissue are in many other parts of the body and are particularly common in the mucous membranes of the airways, lungs, and digestive tract. Smaller amounts of lymphoid tissue are located in the tonsils, adenoids, small intestine (Peyer's patches), and appendix. The lymphoid vessels connect the lymphoid organs with each other and to the other organs.