CD63 and infection: In contrast, platelet responsiveness to stimulation of the PAR4 thrombin receptor did not decrease after infection; platelet CD63 expression was even enhanced at 40 h after infection, relative to baseline, and again irrespective of genotype (p < 0.01 for both genotypes for PAR4AP 0.1 mg/mL and p < 0.001 for PAR4AP 0.2 mg/mL).