The soluble platelet agonists include, thrombin (a product of the coagulation cascade), platelet-activating factor (which is generated under inflammatory conditions), lysophosphatidic acid (which is enriched in atherosclerotic plaques), and thromboxane A2, ADP, and serotonin (which are from the platelets themselves).

What do platelets require to be activated?

Platelets are normally activated in the presence of tissue injury with endothelial disruption and loss of activation inhibitors, exposure of the von Willebrand factor that binds it's receptor and slows circulating platelets, and release of ADP, thrombin, and TxA2 as well as binding of fibrinogen or collagen to αIIb/β3.

What activates platelets during coagulation?

Platelets interact with several coagulation factors, while the coagulation product thrombin is a potent platelet-activating agonist. Activated platelets come in a procoagulant state after a prolonged elevation in cytosolic [Ca2+]i.