Idiopathic thrombocytopenic purpura (ITP) is a hematologic disorder of unknown origin (that is, idiopathic) in which the blood does not clot properly due to a decreased number of platelets (thrombocytopenia). It is characterized by the presence of purpura, small hemorrhages of capillaries that appear as purple bruises on the skin or mucous membranes. There are two forms of the disorder: adult (or chronic) ITP and childhood (or acute) ITP. Adult ITP generally lasts 6 months or longer and usually results from the development of an autoantibody (an antibody formed by an individual against the individual's own tissues) directed against a structural platelet antigen. Childhood ITP generally lasts less than 6 months and is thought to be caused by a viral infection that triggers the synthesis of an antibody that may react with a viral antigen associated with the platelet surface.