Both kinds play a key role in immune responses to infectious microorganisms. B lymphocytes mature in the bone marrow and proceed directly to the lymphoid organs. T lymphocytes mature in the thymus before proceeding to the other lymphoid organs, such as the spleen. Like macrophages, lymphocytes are formed from stem cells in the bone marrow and then circulated in the blood to the lymphoid tissue. The most common cell type in the lymphoid tissue is the lymphocyte. These cells may be fixed in one place, such as lymph nodes, or they may wander in the loose connective-tissue spaces. Macrophages help eliminate invaders by engulfing foreign materials and initiating the immune response. Reticular cells provide structural support, since they produce and maintain the thin networks of fibres that are a framework for most lymphoid organs. Several types of cells are included in the lymphoid system-for example, reticular cells and white blood cells such as macrophages and lymphocytes. Most nodules appear and disappear in response to local needs. Some nodules become relatively permanent structures, such as the tonsils, appendix, and Peyer’s patches, which are in the lining of the small intestine. 10.1055/b-0034-75798 11 Soft Tissue Disease Burgener, Francis A. These are referred to as nodules and are not to be confused with nodes, an entirely different structure. They can establish localized centres of cell production in response to such invasions. In these spaces many cells of the lymphatic system wander and become exposed to invading microorganisms and foreign material. The most diffuse lymphoid tissue is found in the loose connective-tissue spaces beneath most wet epithelial membranes, such as those that line the gastrointestinal tract and the respiratory system.
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