Fibrinogen is a protein synthesized by the liver. It is the substance used in the blood to form a clot. Fibrinogen is cleaved (broken) by the enzyme thrombin to form fibrin peptides (short protein fragments) during normal coagulation (blood clotting). Thrombin also activates fibrin stabilizing factor (Factor XIII) which subsequently cross-links the fibrin into a complex lattice, closing off injured blood vessel walls. Platelet aggregation at the site of injury is facilitated by binding of protein receptors in the platelet cell membranes to fibrin peptides.