Role of VWF and ADAMTS13 in platelet adhesion. VWF multimers may adhere to endothelial cells or to connective tissue in the vessel wall. Platelet GPIb binds to the VWF A1 domain. Flowing blood applies force to the platelets that stretches VWF and exposes a cleavage site for ADAMTS13 in the A2 domain (thin lines in VWF multimers attacked by ADAMTS13). Cleavage of VWF limits the growth of intravascular thrombi. Congenital or acquired ADAMTS13 deficiency allows excessive platelet deposition, causing microvascular thrombosis and TTP.