PS exposure in human platelets requires complement activation. PGN was preincubated for 30 minutes at 37°C with Tyrode buffer (buffer), normal plasma (PGN + plasma) or heat-inactivated plasma (PGN + Δi Plasma) and then added to platelets for 15 minutes at 37°C. Integrin αIIbβ3 and PS expression were detected with (A-C) PAC-1 antibody or (D-F) annexin V, respectively. (G) Normal plasma was pretreated with compstatin, anti-human C5, or C5a (10 μg/mL) for 1 hour before mixing with PGN for 1 hour at 37°C. The mixture was added to platelets in Tyrode buffer and the percentage of annexin V–positive platelets after 15 minutes at 37°C was recorded. The data are representative of 3 independent experiments. *P < .05.