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 the latter to polymerize. The polymerized fibrin together with platelets forms ahemostatic plug or clot over a wound site.

When the lining of a blood vessel is broken, platelets are attracted forming a platelet plug. These platelets express 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 laid down and 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 overlying the platelet plug that completes the clot.