Platelets as immunogens

An immune response to human platelet antigens (HPA), as in neonatal alloimmune thrombocytopenia (NAIT) and post-transfusion purpura (PTP), is the exception rather than the rule and evidence is accumulating for the importance of human leucocyte antigen (HLA) class II restriction in this situation. Platelets have only HLA class I antigens and do not cause primary HLA alloimmunization; in platelet transfusions this is due to the contaminating leucocytes. Autoimmune thrombocytopenia is more common than the alloimmune conditions. The main target antigens for platelet autoantibodies are glycoproteins (Gp) IIb/IIIa and Ib/IX. The mechanism of drug-induced immune thrombocytopenia has been re-examined in relation to the target cell orientation of the antibody and of the drug.