PGN binding to and stimulation of platelets requires IgG. PGN-FITC was incubated with Tyrode buffer (buffer), human plasma (plasma), Ig-depleted plasma, or IgG-depleted plasma + 500 μg/mL human IgG. Platelets were washed free of plasma and incubated in the presence of 1 µM tirofiban with the PGN opsonized with plasma for 30 minutes before staining for Cy5-CD41. The percentage of CD41⁺ platelets binding to PGN-FITC was determined by flow cytometry. The result is representative of 4 experiments. (E) Platelets were prepared and treated with PGN opsonized as described in panels A through D. The activation of platelets was detected by staining with PAC-1 or annexin V. The data are the mean and SE of four experiments, each with a different donor for cells and plasma. *P < .05.