The occurrence in an organism of an immune response to one of its own tissues, that is, a response to a self constituent. Foreign substances to which an organism makes a protective immune response are called nonself. Efficient discrimination between self and nonself, the basis of normal immune function, depends upon a function known as immune tolerance, which means an inertness to substances that could be capable of provoking an immune response. Failure of immune tolerance to self constituents results in an autoimmune response which is often, although not invariably, associated with autoimmune disease. Autoimmune disease occurs when the autoimmune response to self constituents has damaging effects of a structural or functional character.