Fibrin (also called Factor Ia) is a fibrous, non-globular protein involved in the clotting of blood. It is formed from fibrinogen by the protease thrombin, and is then polymerised to form a "mesh" that forms a hemostatic plug or clot (in conjunction with platelets) over a wound site.
Fibrin is involved in signal transduction, blood coagulation, and platelet activation.
Read more about Fibrin: Role in Disease, Physiology, Structure