Coagulation or blood clotting relies on, in addition to the production of fibrin, interactions between platelets.  When the endothelium or the lining of a blood vessel is damaged, connective tissue including collagen fibers is locally exposed. Initially, platelets stick to the exposed connective tissue through specific cell-surface receptors. This is followed by platelet activation and aggregation in which platelets become firmly attached and release chemicals that recruit neighboring platelets to the site of vascular injury.  A meshwork of fibrin then forms around this aggregation of platelets to increase the strength of the clot.