The initial tethering of platelets at sites of vascular injury is mediated by a receptor complex of glycoproteins 1b, IX and V (GP1b-IX-V - frequently referred to as the GPIb receptor). The GP1b component binds to von Willebrand factor (vWF) complexed with collagen exposed in vascular epithelium following injury. In conditions of high shear stress, when a blood vessel is partially blocked, vWF can bind to GP1b:V:IX in tha absence of collagen, a major factor in heart attack and stroke. GPIb-IX-V interaction with vWF:collagen potentiates the ability of alphaIIb betaIII integrin to bind vWF and fibrinogen, triggering stable platelet adhesion and generation of further signals that lead to aggregation.