I nitiatin g adhe sion re ceptor s These receptors reco gnize insoluble adhesive proteins that become accessible to platelets after disruption of the endothe-lial barrier. The subendothelial adhesive proteins mediating initial adhesion include collagen, subendothelial matix-bound von Willebrand factor (vWF), fibronectin and possibly other subendothelial mahix components. The initiating adhesion receptors include glycoprotein Ib-IX (GPIb-IX), which functions as a vWF receptor, integrin crzpr, which functions as a collagen receptor, and other platelet collagen receptors such as glycoprotein IV (GPIV) and glycoprotein VI (GPVI, p62); and other Br integrins such as crspr a fibronectin receptor and (kpt, a laminin receptor. GPIb-IX and collagen recepto (s) initiate signals that lead to platelet activation. For a compre-hensive review of GPIb-IX vWF interactions see the chapters by Clemetson in this volume.

Re sponsive adhe ston re ceptor s This group of platelet adhesion receptors includes the integrin cruupr (GPIIb-IIIa) which is the most abundant platelet adhesion receptol. ouup3 is a receptor for fibrinogen, fibronectin, vitronectin, vWF and thrombospondin, and mediates platelet ag $ egation, firm adhesion, and spreading. In addition, P-selectin (GMP140), which binds glycoprotein ligands such as PSGL-1 and mediates adhesion of activated platelets to white blood cells, appears on the platelet surface only after activation. The ligands for this group of adhesion receptors usually are present in circulating blood, but the interaction with their receptors are regulated either by altering receptor affinity (in the case of anupr) or by translocating the receptor to the platelet surface (P-selectin).