Although the immune system has an elaborate system of checks and balances to ensure self tolerance, occasionally this system breaks down. When the immune system attacks host components causing pathological change, this is called autoimmunity .
Many people experience an autoimmune reaction during their lifetime.
It is these latter conditions where serious immunopathology occurs which are usually considered autoimmune disease. Autoimmune diseases form a spectrum ranging from organ-specific conditions in which one organ only is affected to systemic diseases in which the pathology is diffused throughout the body. The extremes of this spectrum result from quite distinct underlying mechanisms, but there are many conditions in which there are components of both organ-specific and systemic damage. Almost all patients presenting with autoimmune conditions have some autoantibodies present in their serum. It is not always known whether the autoantibodies play an important role in the disease or are a secondary result of the tissue damage which has been caused by the disease process itself.