Fibrin

Fibrin is the active form of coagulation factor I. It is formed by the proteolytic action of thrombin on fibrinogen (inactive form of coagulation factor I). Thrombin cleaves fibrinogen into fibrin-monomer and small fibrinopeptides that have no coagulation activity. The fibrin-monomer molecules then spontaneously polymerize to form fibrin fibers, which form the basis of a blood clot.

Source
ŠVÍGLEROVÁ, Jitka. Fibrin [online]. Poslední revize 2009-02-18, [cit. 2010-11-13]. .