The body’s tolerance of the antigens present on its own cells, i.e., autoantigens or self-antigens. It is theorized that autoreactive T lymphocytes are destroyed in the thymus by negative selection or in peripheral blood. Autoreactive T cells that escape destruction in the thymus may become tolerant because they are exposed to thousands of autoantigens as they circulate in the blood.
The loss of self-tolerance is believed to be due to many hereditary and environmental factors and occurs when autoantigens are damaged, when they link with a foreign antigen, when the structure of an autoantigen is very similar to that of a foreign antigen (molecular mimicry), or when autoreactive T cells are not adequately controlled or are activated by nonspecific antigens. The changes in the appearance of the autoantigen or activation of autoreactive T-cells result in autoantigens being perceived as foreign. Inflammation and destruction of the tissues bearing the antigen occur because of the production of autoantibodies by B cells or the cytotoxicity of autoreactive T cells, which attack the autoantigens.