4 Immune-mediated thrombocytopenias: basic and immunological aspects

Abstract

Acute idiopathic or autoimmune thrombocytopenic purpura (AITP) is a disorder found mainly in children, usually preceded by a viral infection, with a higher incidence in the autumn and winter. The platelet-specific autoantibodies in acute childhood AITP are more often of the IgM class. Chronic AITP occurs mostly in adults. The platelet immunofluorescence test (PIFT) detects platelet-specific autoantibodies with a sensitivity of 65–75%. The autoantibodies in chronic AITP are classified as IgG in 95%, IgM in 26% and IgA in 4% of cases. The antibodies are usually bound to platelets and are detectable as free circulating antibodies in about 40%. AITP in pregnancy may cause neonatal AITP by autoantibodies of the IgG class which pass the placenta barrier. The rare neonatal alloimmune thrombocytopenic purpura (NAITP) are caused by IgG alloantibodies against HPA-1a in 75–90%, HPA1b in 3–5%, HPA 3a in 4–5%, HPA5b in 6–19% and against private platelet antigens in 3%. To confirm the diagnosis of NAITP requires extensive serological testing of the child, and the parents have to be typed for the important platelet-specific antigens by PIFT, monoclonal antibody immobilisation of platelet antigens (MAIPA) and/or enzyme-linked immunosorbent assay (ELISA) techniques. Three mechanisms of drug-induced thrombocytopenias are described. Platelets of both the donor and the patient are destroyed in post-transfusion thrombocytopenic purpura (PTP) but PTP does not occur again if incompatible platelets are re-administered.