Shear-induced platelet aggregation.

Two platelet mechanisms contribute to haemostasis and thrombosis. (1) Compounds such as thrombin activate glycoprotein IIb/IIIa; fibrinogen is the ligand. The cyclooxygenase pathway is involved and so this process is aspirin sensitive. (2) Shearing forces alone activate a different domain on glycoprotein IIb/IIIa; von Willebrand's factor is the ligand. This process is probably non-enzymatic and is aspirin insensitive. The prevention of shear-induced platelet activation may prove to be more rewarding therapeutically than inhibition of aspirin sensitive pathways.