Autoimmunity

Autoimmunity is a state in which the adaptive immune system attacks the body's own molecules, cells or tissues as if they were foreign. Normally the immune system is tolerant of self: T cells that react strongly to self proteins are eliminated while they mature in the thymus, and regulatory T cells hold many of the remaining self-reactive cells in check. When this tolerance breaks down, autoantibodies (antibodies against self) or self-reactive T cells cause damage, mostly through inflammation.

Most autoimmune diseases have no single known cause. They appear to arise from a combination of inherited susceptibility and environmental triggers such as infections, medicines or chemicals. One proposed mechanism is molecular mimicry, in which a microbe carries a protein resembling a human one, so the response against the microbe also strikes the body's own tissue.

Some autoimmune diseases target a single organ. In type 1 diabetes, T cells destroy the pancreatic cells that make insulin, and in Graves disease an antibody overstimulates the thyroid gland. Others are systemic and affect many tissues, as in systemic lupus erythematosus and rheumatoid arthritis. Treatment usually aims to calm the immune response, often with immunosuppressive drugs or corticosteroids.