Von Willebrand's Disease (VWD) is a deficiency or dysfunction of von Willebrand Factor (VWF). VWF is a large multimeric glycoprotein normally found in plasma. This protein is produced in the endothelium, megakaryocytes (stored in the alpha granules of platelets), and subendothelial connective tissue. It can be considered a coagulation factor, but unlike the numbered coagulation factors, it is not an enzyme.
The primary functions of VWF are:
- VWF binds and stabilizes factor VIII. VWF is normally found in the circulation bound to factor VIII and is inactive. Thrombin, like factor IX, can activate factor VIII. When this happens, VWF is released from factor VIII. In the absence of VWF, factor VIII degrades quickly; its half-life is only 1–2 hours as opposed to 8–12 hours when bound to VWF.
- VWF binds to collagen when it is exposed due to injury. The endothelium also releases VWF itself when damaged. VWF binds to glycoprotein Ib receptor on platelets, anchoring them to the site of damage. VWF also binds the GP IIB/IIIa receptor on platelets which binds platelets to each other (see figure).
