The immune system specifically recognizes and eliminates foreign antigens and thus protects the integrity of the host. During maturation of the immune system, tolerance mechanisms develop that prevent or inhibit potentially harmful reactivities to self-antigens. Autoreactive B and T cells that are generated during immune responses are eliminated by apoptosis in the thymus, lymph nodes, or peripheral circulation or are actively suppressed by regulatory T cells. However, autoreactive cells may survive because of failure of apoptosis or molecular mimicry, that is, presentation and recognition of cryptic epitopes of self-antigens or aberrant lymphokine production. Development of immune responses and tolerance is determined by an interplay of genetic and environmental factors. Autoimmunity is a result of the breakdown of one or more of the mechanisms of immune tolerance.