Acquired Immunity

The primary lymphoid organs are the bone marrow and thymus. All immune stem cells originate in the bone marrow. One population of stem cells remains in the bone marrow and differentiates into mature naive B cells while another population migrates to the thymus and differentiates into mature naive T cells. Each cell type is capable of responding to an antigen but has not yet received the stimulus required to elicit a reaction. Once these stem cells have differentiated into mature naive effector cells capable of responding to antigen, they leave the primary lymphoid organs and migrate throughout the body constantly circulating through the secondary lym-phoid tissues. The secondary lymphoid organs consist of the spleen, lymph nodes, and MALT (mucosal-associated lymphoid tissue) that filter the blood, extracellular fluid, and substances crossing mucosal surfaces respectively for foreign antigens. Secondary lymphoid organs are sites where immune responses often begin because of their ability to concentrate antigen.

Acquired immunity is specific. The adaptive responses can be separated into two divisions, humoral and cell-mediated immunity. The essential functions of the two divisions are carried out primarily by B cells in the humoral response and T cells in the cell-mediated response. Antibodies produced by terminally differentiated B cells can discriminate between different antigens and T cells can discriminate between different processed antigens as well as between self and nonself proteins. The acquired system can also recruit the mechanisms of innate immunity to amplify its own responses. Once B cells have gone through the various precursor stages in the bone marrow, their specificity is determined by antibodies expressed on their surface that bind to complementary antigens. T cell specificity is determined by T cell receptors (TcR) on the cell surface. The TcR can only bind processed antigens that are presented to it in association with a major histocompatability complex (MHC) protein. In the thymus T cells go through numerous precursor stages and those cells that bind MHCs too strongly or with too low of an affinity are deleted. In addition, those cells that recognize processed self antigens are deleted as well. Deletion of these cells limits autoimmune reactions and is referred to as tolerance. Other tolerance mechanisms that occur outside the primary lymphoid organs are referred to as peripheral tolerance.

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