Immune-mediated thrombocytopenias: basic and immunological aspects

Acute idiopathic or autoimmune thrombocytopenic purpura (AITP) is a disorde r found mainly in children, usually preceded by a viral infection, with a h igher incidence in the autumn and winter. The platelet-specific autoantibod ies in acute childhood AITP are more often of the IgM class. Chronic AITP o ccurs mostly in adults. The platelet immunofluorescence test (PIFT) detects platelet-specific autoantibodies with a sensitivity of 65-75%. The autoant ibodies 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 detecta ble as free circulating antibodies in about 40%, AITP in pregnancy may caus e 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-la in 75-90%, HPA1b in 3-5%, HPA 3 a in 4-5%, HPA5b in 6-19% and against private platelet antigens in 3%. To c onfirm the diagnosis of NAITP requires extensive serological testing of the child, and the parents have to be typed for the important platelet-specifi c 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 b oth the donor and the patient are destroyed in post-transfusion thrombocyto penic purpura (PTP) but PTP does not occur again if incompatible platelets are re-administered.