Thrombin is a substance (enzyme) in the bloodstream that is needed for blood to clot. When a person is cut or wounded, thrombin and a protein called fibrinogen make a stringy material that traps blood cells and then gradually decomposes as the area heals.
Only thrombin located at the area of the injury is activated, and only for a few seconds. This process helps prevent a potentially dangerous blood clot, called a thrombus, from forming and traveling through the bloodstream.
WebMD Medical Reference from Healthwise