IAV H₁N₁ activation of platelets is dependent on the presence of serum. Human platelets (100 × 10⁶/mL) bathed in 2% HS were incubated with saline (control) or stimulated with IAV H₁N₁ for 20 minutes. (A) Expression of the active form of integrin α2β3 (PAC-1 binding) and CD62P on CD41⁺ platelets analyzed by flow cytometry. (B) Representative forward scatter photomultiplier tube (FSC-PMT) and side scatter (SSC) portrayals of the CD41⁺ events detected upon treatment with IAV. The relative dimensions (in nanometers) of the MPs are presented according to size-defined microsphere calibrations. Kinetics of platelet activation by IAV. Platelets (100 × 10⁶/mL) incubated in 2% HS were incubated with vehicle or stimulated with IAV H₁N₁ for varying periods of time (0.5-20 minutes), after which expression of the active form of integrin α2β3 (PAC-1 binding) and CD62P (C), MP formation (D), and 12-HETE synthesis (E) were measured. Serum is required for platelet activation. Washed human platelets (100 × 10⁶/mL) were incubated without or with IAV H₁N₁ in the absence or in the presence of 2% autologous serum. After 20 minutes, the expression of the active form of integrin α2β3 (PAC-1 binding) and CD62P (F), MP formation (95% confidence interval, 7473-428 855) (G), and 12-HETE synthesis (H) were measured. (F-H) N = 35; data are mean + standard error of the mean (SEM). ***P < .0001.