Any disease caused by an immune response (see immunity ) against antigens in the tissues of one's own body.
The immune system has two known ways to prevent such a response: destruction of lymphocyte s in the thymus before they leave to attack one's own tissues and loss of ability to react to their target antigens by any such cells that do leave the thymus. Autoimmune diseases arise when these mechanisms fail and lymphocytes destroy host tissues; examples include type 1 diabetes mellitus , systemic lupus erythematosus , pernicious anemia , and rheumatoid arthritis . Treatment may replace the function of the affected tissue (e.g., insulin therapy for diabetes) or suppress the immune system (see immunosuppression ). Allergy is another type of autoimmune reaction.