Fibrin (also called Factor Ia)'s a fibrous protein involved in the clotting of blood, and's non globular. It's a fibrillar protein that's polymerised to form a "mesh" that forms a hemostatic plug or clot (in conjunction with platelets) over a wound site. Fibrin's made from fibrinogen, a soluble plasma glycoprotein that's synthesised by the liver. Processes in the coagulation cascade activate the zymogen prothrombin to the serine protease thrombin, which's responsible for converting fibrinogen… (More on Fibrinogen)