Many antigen systems are present on platelet membranes, including ABO, HLA, and PLA1. Antibodies to platelets may be directed to any of these antigens and may be either alloantibodies or autoimmune antibodies. Alloantibodies are antibodies that a person makes against foreign antigens, usually after transfusion or pregnancy. Autoantibodies are formed against a persons own platelets. This can occur as a primary disorder, such as idiopathic thrombocytopenic purpura (ITP) or secondary to other diseases, such as lymphoma. Both allo and autoantibodies can cause thrombocytopenia. Tests for platelet antibodies are useful in the following clinical situations: Platelet refractoriness after multiple transfusions, post-transfusion purpura, neonatal isoimmune purpura, drug induced platelet antibodies, and idiopathic thrombocytopenic purpura (ITP). Symptoms of ITP include increased bruising, heavy menstrual periods, retinal hemorrhages, gastrointestinal bleeding, normal coagulation tests, and gingival (gum) bleeding.