Previously called antithrombin III (abbreviated AT III), human antithrombin is one of the major physiological inhibitors of coagulation. A natural serine protease inhibitor, antithrombin acts mainly on thrombin (IIa) and activated factor X (FXa), as well as on the activated forms of factors IX, XI and XII. This reaction is catalyzed by heparin. The normal level of antithrombin is between 80 and 120% in adults and it is about half in newborns. Antithrombin deficiency predisposes to thrombosis.