Von Willebrand Factor (VWF) is a large, multimeric glycoprotein that circulates in the blood. It has two major functions:
  1. To bind to Factor VIII and protect it from being chewed up by proteases in the bloodstream
  2. To play a role in blood clotting, by binding platelets to a site of injury
It is produced in the body by only two types of cells: endothelial cells, which line blood vessels, and megakaryocytes, which produce platelets. A certain amount of VWF is secreted constitutively into the blood stream; the rest is stored in special compartments in endothelial cells and platelets, and secreted in response to inflammation or injury.

Endothelial cells are held in place by a collagen matrix, so when a blood vessel is injured, this collagen is exposed to the blood. The injury triggers nearby endothelial cells to dump their stored VWF into the blood, which combines with the VWF already there. All this VWF binds to the the collagen and other exposed proteins.

Once stuck there, it has a very high affinity for certain proteins on the surface of platelets. So when platelets come floating along in the blood, they get snagged by the VWF (adhesion) and clump together (aggregation). This is the first step in blood clotting. Next, a chemical cascade is triggered which involves various blood factors, including Factor VIII, the one missing in the most common type of hemophilia. This ultimately forms insoluble strands of fibrin which seal off the damaged area to prevent blood loss.

Why do we care about VWF? Well, some people either don't produce enough of it, or the stuff they produce doesn't function properly. These people have a bleeding disorder called von Willebrand's disease (VWD). The most common type of VWD is quite mild, and many people have it without realising it, but there are rare types which can be extremely severe. Since VWF protects Factor VIII, people with severe VWF also end up with pretty bad hemophilia -- a sort of double whammy!

Finally, although most VWF research focuses on what happens when you don't have enough of it, recent studies suggest important consequences for having a little too much. As might be expected, having elevated VWF levels increases the chance of forming clots when they're not needed, leading to problems such as thrombosis, heart attack, and stroke.

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