The binding strength of KKO (red) vs RTO (green) to PF4 or PF4/H complexes coated on different surfaces. (A) When PF4 (top) or PF4/H complexes (bottom) were immobilized on a gold surface, the reaction between KKO (red) and PF4 is weaker (rupture force up to 100 pN) than that between KKO and PF4/H complexes (rupture force up to 200 pN), while RTO (green) shows much weaker interactions, as evidenced by the low binding “counts.” (B) When PF4 (middle) or PF4/H complexes (bottom) were coated on platelet surfaces, both KKO (red; rupture force up to 800 pN) and RTO (green; rupture force up to 200 pN) show stronger interaction forces than on the solid phase (A) or on noncoated platelets (top). The broad distribution of binding forces indicates that the binding site of KKO is presented very variably, allowing weak to very strong binding of KKO.