Effect of endothelial cells on binding of PF4 to platelets and WBCs. Whole blood from PF4^null mice was flowed through a microfluidic channel coated with HUVECs that had been either previously unexposed to PF4 (A) or previously incubated with 100 µg/mL of hPF4 (B). (A-B) The infused whole blood was also preincubated with 0, 10, or 100 µg/mL of PF4, as indicated. Shown is relative cell-surface HIT antigenicity detected using KKO on platelets (red circles) or WBCs (yellow squares) after traversing the channel compared with results before entering the channel. N = 6 separate studies per arm, each in duplicate. *P < .01, **P < .001 with values determined using analysis of variance of calculated values against a hypothetical value 1 = no change (dashed horizontal line).