Binding of platelets to hPF4-VWF-HIT antibody complexes is dependent on both their FcγRIIA and GPIb/IX receptors. (A) As in Figure 4B, representative fields showing platelet accumulation on injured endothelium after exposure to the flow of recalcified human blood in the presence or absence of PF4 and KKO and the GPIb-IX-blocking antibody AK2 or FcγRIIA-blocking antibody IV.3 or both. (B) In each case, binding of platelets is shown relative to injured endothelium with added KKO only as the comparator. Means ± 1 SEM are shown for N ≥ 5 separate injured studies. P values were determined using a 1-way ANOVA. See supplemental Figure 5 for additional controls.