lymphoid tissue

Cells, tissues, and organs composing the immune system, including the bone marrow, thymus, spleen, and lymph nodes.

The most highly organized components are the thymus and lymph nodes, and the least organized are the cells that wander in the loose connective-tissue spaces under membranes lining most body systems, where they can establish lymph nodules (local lymphocyte production centre) in response to antigens. The most common lymphoid tissue cell is the lymphocyte. Others are macrophages, which engulf foreign materials and probably alter them to initiate the immune response, and reticular cells, which produce and maintain thin networks of fibres as a framework for most lymphoid organs. See also immunity; lymphatic system.

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▪ anatomy

 cells and organs that make up the lymphatic system, such as white blood cells (leukocytes (leukocyte)), bone marrow, and the thymus, spleen, and lymph nodes (lymph node).

      Lymphoid tissue has several different structural organizations related to its particular function in the immune response. The most highly organized lymphoid tissues are in the thymus and lymph nodes, which are well-defined encapsulated organs with easily identifiable architectures. In the spleen (a soft, purplish organ lying high in the abdomen), the lymphoid tissue is a cylinder of loosely organized cells surrounding small arteries. In the bone marrow this tissue is mixed with the blood-forming cells, and no organization is apparent. The most diffuse lymphoid tissue is found in the loose connective-tissue spaces beneath most wet epithelial (epithelium) membranes, such as those that line the gastrointestinal tract and the respiratory system. In these spaces many cells of the lymphatic system wander and become exposed to invading microorganisms and foreign material. They can establish localized centres of cell production in response to such invasions. These are referred to as nodules (lymph nodule) and are not to be confused with nodes, an entirely different structure. Some nodules become relatively permanent structures, such as the tonsils (tonsil), appendix, and Peyer's patches (Peyer patch), which are in the lining of the small intestine. Most nodules appear and disappear in response to local needs.

      Several types of cells are included in the lymphoid system—for example, reticular cells and white blood cells such as macrophages and lymphocytes (lymphocyte). Reticular cells provide structural support, since they produce and maintain the thin networks of fibres that are a framework for most lymphoid organs. Macrophages (blood) help eliminate invaders by engulfing foreign materials and initiating the immune response. These cells may be fixed in one place, such as lymph nodes, or they may wander in the loose connective-tissue spaces. The most common cell type in the lymphoid tissue is the lymphocyte. Like macrophages, lymphocytes are formed from stem cells in the bone marrow and then circulated in the blood to the lymphoid tissue. T lymphocytes mature in the thymus before proceeding to the other lymphoid organs, such as the spleen. B lymphocytes mature in the bone marrow and proceed directly to the lymphoid organs. Both kinds play a key role in immune responses to infectious microorganisms.

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