Thrombin is a blood enzyme that converts fibrinogen to fibrin in the process of blood clotting. It is formed from its precursor molecule, prothrombin, which is produced by the liver and released into the bloodstream. When an injury occurs and a blood vessel is damaged, a series of chemical reactions in the blood occur that ultimately lead to the formation of thrombin. Once formed, thrombin acts on a protein in the blood called fibrinogen, converting it into fibrin, which forms a clot that helps to stop bleeding. Thrombin is also involved in other biological processes, such as wound healing and inflammation.