Platelets respond rapidly to alterations of endothelial cells by attaching firmly to the site of lesion, where exposure of subendothelial components may have occurred. The first layer of platelets is in contact with the thrombogenic surface (adhesion), whereas subsequent growth of the hemostatic plug depends on platelet-to-platelet interactions (aggregation). Both aspects of platelet function are influenced by von Willebrand factor and fibrinogen interaction with specific platelet membrane receptors. Multiple domains of von Willebrand factor and fibrinogen are involved in securing initiation and growth of platelet thrombi.