mAb R6H8 is a potent inhibitor of clot retraction, but potency requires bivalent binding. (A) Washed platelets (3 × 10⁸ platelets per mL) were incubated with the indicated concentration of mAb R6H8 or 7E3 IgG for 20 minutes at room temperature. Clot retraction was initiated by adding platelets treated as above to an aggregation cuvette containing 2 mM CaCl₂ and 0.2 U/mL thrombin, as indicated in “Methods.” Shown are representative images of 4 independent experiments. (B) Washed platelets (3 × 10⁸ platelets per mL) were incubated with the indicated concentration of mAb R6H8, R6H8 F(ab')₂, or R6H8 Fab for 20 minutes at room temperature. Clot retraction was initiated by adding platelets treated as above to an aggregation cuvette containing 2 mM CaCl₂ and 0.2 U/mL thrombin. Shown are representative images of 3 independent experiments. Cntl, control (untreated) platelets.