the process whereby platelets adhere to collagen released from endothelial cells in damaged blood vessels and aggregate to form a plug. An important metabolic pathway in platelets converts membrane phospholipids to *thromboxane A2 (TXA2), which can activate platelets. ADP, released from platelet granules, and *thrombin are other activators. The adhesive and aggregation reactions of platelets are mediated through surface membrane glycoproteins (Glp), Glp Ib and Glp IIb/IIIa. Aspirin acts as an *antiplatelet drug by irreversibly inhibiting one of the enzymes of this pathway, cyclo-oxygenase. Clopidrogel inhibits the ADP receptor on platelet membranes, and *abciximab blocks platelet aggregation by inhibiting Glp IIb/IIIa (see illustration overleaf).
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