MAINTENANCE OF SELF TOLERANCE
Environmental triggers, such as sunlight, drugs/chemicals, and infectious agents, act on a genetic background that regulates tolerance to self. The immune system has evolved a remarkable ability to distinguish self from nonself. Immune tolerance is achieved by multiple mechanisms, operating both centrally and peripherally. Central tolerance occurs during the development of T and B lymphocytes in the thymus and bone marrow, respectively. This mostly involves the deletion of autoreactive cells before they exit the primary lymphoid organs. In general, lymphocytes exhibiting strong reactivity for ubiquitously expressed self-antigens are deleted in this manner, whereas autoreactive cells of lower affinity for self may escape central tolerance. These cells are held in check by peripheral tolerance mechanisms.