Immune complex formation as the trigger for aggregation. Amplitude of the aggregation at minute 10 of washed normal platelets in the absence (−) or presence (+) of ristocetin (0.17 mg/mL), vWF (10 μg/mL), ion-exchange chromatography–isolated patient Igs (0.5 mg/mL), the anti-vWF antibody AJvW-2 (20 μg/mL), the anti-GPIb antibody G19H10 (20 μg/mL), or the anti-FcγRII antibody IV.3 (10 μg/mL), as indicated. Platelet mixtures were preincubated for 10 minutes before initiation of aggregation by ristocetin and/or stirring.