Inhibitory effect of PAR3 IgG on mouse platelet responses to α-thrombin and ablation of inhibition by the peptide antigen. α-Thrombin–induced (0.5 nmol/L) platelet aggregation (A) and secretion (B) are shown. IgGs were preincubated with peptide antigens for 5 minutes at 37°C. Platelets were incubated with this mixture (final concentration of IgG, 60 μg/mL; antigen, 0.1 μmol/L) for 5 minutes at 37°C; responses were then analyzed in a lumiaggregometer. α-Thrombin was added at approximately 3 minutes (arrow). Responses were expressed as the percentage of maximum, with 100% defined as the response induced by 10 nmol/L α-thrombin in platelets incubated with preimmune IgG only. The latter had no effect on platelet responses. The data shown were replicated in two separate experiments.