Binding of mAb to human platelets in the presence of added hPF4. (A) The graph shows the fold increase in the mean fluorescence intensity (MFI) of antibody binding in the presence and absence of the noted concentrations of PF4. Gray squares indicate TRA isoimmune control; open diamonds, anti–human CD41 mAb; and filled circles, KKO. Each antibody was added at 50 μg/mL. (B) The fold change in antigenicity for KKO in the presence of PF4 at 12.5 μg/mL (open diamonds), 50 μg/mL (gray squares), and 200 μg/mL (filled circles), with heparin added at the concentrations shown. The y-axis indicates fold change from that at baseline without heparin. (C) Platelet activation by KKO (50 μg/mL) at the indicated PF4 concentrations as measured by annexin V binding. (D) Kinetics of KKO binding (50 μg/mL) in the presence of 50 μg/mL of PF4. The mean ± 1 standard deviation (SD) is shown for the experiments performed 3 times, each in triplicate.