Idiopathic thrombocytopenic purpura (ITP) Primary tabs. Definition. Idiopathic thrombocytopenic purpura (ITP) is a bleeding disorder in which the immune system destroys platelets, which are necessary for normal blood clotting. Persons with the disease have too few platelets in the blood. ITP is sometimes called immune thrombocytopenic purpura or simply, immune thrombocytopenia. Immune thrombocytopenic purpura; ITP; Immune thrombocytopenia. ITP occurs when certain immune system cells produce antibodies against platelets.