Autoimmunity: A misdirected immune response that occurs when the immune system goes awry and attacks the body itself.

Autoimmunity is present to some extent in everyone and is usually harmless. However, autoimmunity can cause a broad range of human illnesses, known collectively as autoimmune diseases. Autoimmune diseases occur when there is progression from benign autoimmunity to pathogenic autoimmunity. This progression is determined by genetic influences as well as environmental triggers.

Autoimmunity is evidenced by the presence of autoantibodies (antibodies directed against the person who produced them) and T cells that are reactive with host antigens.