Platelet aggregation induced by type I collagen fibrils with distinct morphologies. Aggregation at 37°C is shown as an increase in light transmittance. The arrows indicate addition of agonist at the final concentrations indicated. The pH of all collagen preparations was adjusted to 7.4 immediately before adding to platelet-rich plasma. The anti-₂β₁monoclonal antibody was added as indicated at a final concentration of 100 μg/mL, which was shown to produce a maximum specific effect.