Chronic Immune (idiopathic) thrombocytopenic purpura (ITP) is an autoimmune disorder in which antiplatelet autoantibody induces platelet destruction. Platelet surface membrane proteins become antigenic, stimulating the immune system to produce autoantibody. The initial antigenic response probably occurs in the spleen, inducing autoantibody production followed by stimulation of other antibody-producing tissues, particularly the bone marrow. Autoantibodies against either platelet glycoprotein (GP)IIb/IIIa or GPIb/IX are produced by about 75% of ITP patients and can be detected using antigen-specific assays. The spleen is the major site of platelet destruction in ITP because of its unique milieu. About one third of the platelet mass is present in the spleen at all times, where the local production of antiplatelet antibody leads to high autoantibody concentrations. These antibody-sensitized platelets circulate slowly through the phagocytic cell-rich spleen, resulting in their destruction. Since autoantibody binds to both platelets and megakaryocytes, both platelet destruction and inhibition of thrombopoiesis may be of importance in the pathogenesis of chronic ITP.