A cofactor is a chemical substance, which binds to a protein, and which is necessary for the biological activity of the latter. These proteins are often enzymes, and coFactors can be thought of as "helper molecules" aiding in biochemical transformations. VWF is composed of 15 to 20 multimers ranging in molecular weight from 500 kDa to 20,000 kDa and high molecular weight multimers are essential for biological activity. Its role is on the one hand to transport FVIII in the circulation to protect it from its degradation and on the other hand it participates in adhesion and platelet aggregation.