<P> Fibrin (also called Factor Ia) is a fibrous, non-globular protein involved in the clotting of blood . It is formed by the action of the protease thrombin on fibrinogen which causes it to polymerize . The polymerized fibrin together with platelets forms a hemostatic plug or clot over a wound site . </P> <P> When the lining of a blood vessel is broken, platelets are attracted forming a platelet plug . These platelets have thrombin receptors on their surfaces that bind serum thrombin molecules which in turn convert soluble fibrinogen in the serum into fibrin at the wound site . Fibrin forms long strands of tough insoluble protein that are bound to the platelets . Factor XIII completes the cross-linking of fibrin so that it hardens and contracts . The cross-linked fibrin forms a mesh atop the platelet plug that completes the clot . </P> <P> Excessive generation of fibrin due to activation of the coagulation cascade leads to thrombosis, the blockage of a vessel by an agglutination of red blood cells, platelets, polymerized fibrin and other components . Ineffective generation or premature lysis of fibrin increases the likelihood of a hemorrhage . </P>

The factor that is responsible for stabilizing a soluble firbrin monomer into an insoluble clot is