Serological characterization of anti–protamine-heparin antibodies: anti–protamine-heparin antibodies are capable of platelet activation in a heparin-dependent manner via cross-linking FcγIIa receptors. Heat-inactivated sera from patients with anti–protamine-heparin antibodies were incubated with washed platelets in the presence of buffer, heparin (0.2 IU/mL), and protamine (2 µg/mL) with or without heparin (0.2 IU/mL). Inhibition studies were performed with protamine plus high heparin (100 IU/mL) or protamine (2 µg/mL) plus heparin (0.2 IU/mL) in the presence of FcγIIa receptor-blocking antibody (IV.3). Platelet aggregation was determined by change in turbidity of the suspension and assessed every 5 minutes, as described.¹³