Platelet-activating factor mediates procoagulant activity on the surface of endothelial cells by promoting leukocyte adhesion.

Endothelial cells co-express platelet-activating factor and P-selectin on their surfaces after activation by certain receptor-mediated agonists. Together they mediate the adhesion of leukocytes to the endothelial cell surface. P-selectin tethers leukocytes to the endothelial cell surface allowing leukocyte activation by platelet-activating factor. Adhesion and activation are specific for leukocytes because they are the only cells known to express the ligand for P-selectin. Leukocytes adherent to the endothelial cell surface may promote thrombosis by three mechanisms: (1) they secrete factors that damage the underlying endothelium, (2) they secrete factors that directly initiate the coagulation cascade, and (3) they bind and activate platelets.