Von Willebrand factor (vWF) is a self-polymerizing clotting protein present in the serum and the subendothelial basal lamina, which has binding sites for collagen, platelets, and fibrin. At a site of injury, vWF forms the bridge between the exposed collagen fibers and platelets in circulation, stimulating platelet degranulation and initiating the cellular component of the clotting cascade. An equally impoant role for vWF is binding platelets to the newly formed fibrin strands in a blood clot. Factor VIII and Factor IX are clotting proteins of the intrinsic pathway. Factor VIII acts in conce with activated Factor IX (IXa) to cleave Factor X to Xa. Xa is the prothrombin activator central to both the intrinsic and extrinsic pathways. Fibronectin is a serum protein that acts as an opsonin for phagocytic cells in clots. Fibronectin binds non-specifically to bacteria and other materials in the newly forming clots, and draws the cell membrane of phagocytes into contact with these substances. Tissue factor is a protein released from injured tissues that works in conce with Factor VII to initiate the extrinsic pathway of coagulation. Like Factors VIII and IX, tissue factor and Factor VII cleave Factor X to Xa.