Surface PF4/GAG antigenicity on monocytes compared with platelets. (A) PF4/GAG complexes detected using the HIT-like monoclonal antibody KKO on human monocytes (♦) compared with simultaneously studied human platelets from the same blood sample (○) and (B) in the presence of increasing concentration of heparin (0.8-200 μg/mL ≈ 0.17-42.4 USP/mL) at 3 fixed PF4 concentrations: 12.5 μg/mL (left), 50 μg/mL (center), and 200 μg/mL (right). The graphs show the fold-increase in mean fluorescence intensity (MFI) of antibody binding in the presence of the concentrations of PF4 and heparin noted compared with MFI in its absence after the results were normalized for size based on equalizing the background MFI of monocytes and platelets. N ≥ 3 experiments, each done in duplicate. The mean ± 1 SE is shown. *P < .05, comparing the fold increase in MFI for monocytes to platelets at a specific PF4 concentration.