It is generally believed that many T cells escape central tolerance, especially those with the capacity to react with self-antigens not expressed in the thymus. This view is consistent with the relative ease of inducing organ-specific autoimmunity by deliberate immunization with peripheral antigens. While tissue-specific, self-reactive T cells generally ignore their target antigen because it is inaccessible or not presented in an immunogenic form, organ-specific autoimmune diseases are common . In order to minimize autoimmunity mediated by self-reactive T cells in the periphery, the immune system has apparently developed various peripheral tolerance mechanisms that can restrain, quell, or kill such cells. Peripheral tolerance is much more than a backup mechanism to prevent autoimmunity - it is an active process that is probably invoked to limit T-cell response to all antigens.
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