Human von Willebrand factor (factor VIII R:Ag) is a 270 kDa multimeric plasma gylcoprotein. The von Willebrand factor has functional binding domains to platelet glycoprotein Ib, glycoprotein IIb/IIIa, collagen and heparin. The factor is synthesized by endothelial cells and is also present in platelets and megakaryocytes.