Thymic T-Cell Education: Positive & Negative Selection
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چکیده
TOLERANCE (which is antigen-specific) must be distinguished from immune deficiency (non-specific). Its most important manifestation, the maintenance of SELFTOLERANCE, was originally explained by the Clonal Selection theory as the result of CLONAL ABORTION of self-reactive clones. However, potentially self-reactive Tand B-cells do exist in normal individuals, leading to the recognition of the importance of active processes in the maintenance of tolerance, such as SUPPRESSION. The dynamic and competing balance between immunity and tolerance is discussed, together with three useful model systems, INDUCED CLONAL ABORTION, RECEPTOR BLOCKADE, and ANTIGEN SEQUESTRATION. Two general categories of tolerance are recognized, CENTRAL TOLERANCE which is generated during development of T-cells within the thymus, and PERIPHERAL TOLERANCE generated by a variety of mechanisms outside this central lymphoid organ.
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