Organs of the Immune System
Primary immune organs (thymus and bone marrow) are the sites of lymphocyte development and maturation. The creation of the antigen-specific receptors of the lymphocytes occurs during development in the primary organs before the lymphocyte encounters antigen. B-cell development occurs in the bone marrow; T-cell development begins in the bone marrow and finishes in the thymus.
Secondary immune tissues are the sites throughout the body where antigen-specific lymphocytes contact antigens. Antigens enter the different types of secondary lymph tissues via different mechanisms. Both B and T cells circulate throughout the body, passing through the secondary lymphoid tissue. Lymphocyte circulation is influenced by chemokines and cell surface receptors.
Each secondary lymph tissue has T- and B-cell zones for activation and differentiation of lymphocytes. Upon immune stimulation, activation of appropriate B and T cells will lead to the formation of germinal centers, which are areas of intense B-cell proliferation and differentiation.
- Lymph nodes throughout the body are connected via lymphatic vessels. Fluid (lymph) from extravascular spaces is collected in the lymphatic vessels and returned to the circulation via the left subclavian vein. The naive lymphocytes enter the lymph node via the blood vessels and migrate to their respective B- and T-cell zones. Free antigens from the tissue, dendritic cells (with processed antigen), and immune complexes enter the lymph node in the lymph. Lymphocytes that recognize specific antigens (either free antigens in the case of B cells or presented with appropriate MHC in the case of T cells) will proliferate and differentiate into effector lymphocytes.
- The spleen is primarily responsible for surveillance of the blood. Antigens that arrive in the blood activate B cells and T cells for proliferation and differentiation in the same manner as for the lymph node.
- The mucosal epithelium of the respiratory and the gastrointestinal tracts are particularly vulnerable to infection and contain specialized secondary lymph tissue broadly referred to as mucosa-associated lymphoid tissue (MALT). Gut-associated lymphoid tissues (GALTs) include the tonsils, adenoids, and Peyer patches. Antigens are delivered across the mucosa and into the MALT by specialized epithelial cells, such as M cells (microfold cells).
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