Effect of antithymocyte globulin and other immune reactants on human platelets

Human platelets suspended in autologous plasma do not respond to nonspecific immune complexes as do platelet suspensions from rabbits and dogs. However, platelets of all three species do undergo aggregation and the release reaction when exposed to antibodies directed against platelets. Antithymocyte globulin (ATG) contains such antibodies, apparently because of antigens common to both thymocytes and platelets. ATG-induced platelet aggregation and release is thus a specific reaction which may be responsible for the thrombocytopenia and thrombotic complications occasionally seen following the administration of ATG. However, if ATG is given properly, its effect on platelets should not constitute a contraindication to the use of this immunosuppressive drug. Since nonspecific immune complexes do not affect human platelets in the presence of plasma, it would appear that platelet aggregates seen in hyperacute and acute rejection result from endothelial damage rather than an effect of immune complexes on platelets.