A disorder of the body’s defence mechanisms in which an immune response is elicited against its own tissues, which are thereby damaged or destroyed. Effector cells of the immune system called T cells bind to self antigens and elicit the production of antibodies or a cytotoxic response. For example, insulin-dependent diabetes mellitus is caused by an immune response against proteins in the B cells of the pancreatic islets, thus abolishing the production of insulin. Rheumatoid arthritis, systemic lupus erythematosus, myasthenia gravis, and several forms of thyroid dysfunction are other examples of autoimmune diseases. Autoimmunity arises from a breakdown in immunological tolerance to self tissues and may also involve a failure of regulatory T cells to suppress immune responses to self antigens.