Idiopathic thrombocytopenic purpura (ITP) is an autoimmune disease characterized by increased platelet clearance caused by anti-platelet autoantibodies, which bind to circulating platelets resulting in destruction. by the reticuloendothelial system. We have recently developed enzyme-linked immunospot assay to detect circulating B cells secreting anti-platelet antibody. An increase in anti-platelet antibody-producing B cells in peripheral blood was specifically detected in ITP patients, but in none of thrombocytopenic patients without ITP or healthy donors. While earlier studies reported the presence of platelet-reactive T cells in ITP patients, we have recently found that GPIIb-IIIa is one of major target antigens recognized by platelet-reactive CD4+ T cells. Since GPIIb-IIIa-reactive CD4+ T cells had helper activity promoting production of anti-platelet antibody, these autoreactive T cells are involved in production of pathogenic anti-platelet autoantibody in ITP patients. Suppression of GPIIb-IIIa-reactive CD4+ T cells may be of therapeutic use in treating refractory patients.