Immune thrombocytopenia (THROM-bo-si-toe-PE-ne-ah), or ITP, is a bleeding disorder. In ITP, the blood doesn’t clot as it should. This is due to a low number of blood cell fragments called platelets (PLATE-lets) or thrombocytes (THROM-bo-sites). Also referred to as idiopathic thrombocytopenia purpura.