Integrin α_IIbβ₃ is required for platelet aggregate formation but not for primary adhesion of platelets to collagen under flow ex vivo. (A) Percent surface coverage area of platelets from α₂β₁^–/–, GPVI-FcRγ^–/–, and wild-type mice after being treated with 30 μg/mL blocking anti-α_IIbβ₃ antibody Leo.H4 (□) or control rat IgG (RatIgG; ▪) at 1300 s^–1. The results shown were determined by image analysis of the phase-contrast images and are the mean ± SEM (n = 4). Statistical significance was seen between treated and untreated wild-type platelets (P < .01). (B) Representative phase-contrast images after flow.