Mouse platelet activation by IAV H₁N₁ requires FcγRIIA. (A) Surface expression of FcγRIIA (CD32a) on WT (left), Tg-FcγRIIA (middle), and human platelets (right) was determined by incubating platelets with V450-labeled anti-CD41 and phycoerythrin (PE)–conjugated anti-CD32a antibodies followed by fluorescence-activated cell sorter analysis. (B-C) Platelets from WT and Tg-FcγRIIA mice were incubated with 3% serum from influenza naïve, H₁N₁-immune, or H₃N₂-immune mice in the absence or in the presence of IAV H₁N₁ and IV.3 mAb. After 60 minutes, 12-HETE (B) and MP (C) production were determined. N = 5; data are mean + SEM. **P < .001; ***P < .0001.