Platelet aggregates, stabilized by fibrin, rapidly form hemostatic plugs when blood vessels are severed or arterial thrombi at sites of vessel injury, such as ruptured atherosclerotic plaques, or regions where blood flow is disturbed, such as at stenoses. These thrombi cause the thromboembolic complications of atherosclerosis: heart attacks, strokes, and peripheral vascular disease. Platelet adhesion to subendothelial components such as collagen activates signalling pathways that lead to thromboxane A2 formation and secretion of platelet granule contents, including ADP. Both these substances cause platelet aggregation, a process in which the integrin, glycoprotein IIb/IIIa, becomes a receptor for fibrinogen, which forms bridges between adjacent platelets. On the surface of stimulated platelets, coagulation is accelerated and thrombin is generated; it is a potent inducer of platelet aggregation and secretion and also causes fibrin to form around the aggregates, stabilizing them. There are receptors on the platelet surface for thrombin, thromboxane A2, collagen, ADP, platelet-activating factor, fibrinogen, von Willebrand factor, and other ligands. Agents that inhibit platelet aggregation and the signalling pathways that are activated by the various aggregating agents are under intensive investigation in many laboratories.