Antithrombin Proteins: An endogenous family of proteins belonging to the serpin superfamily that neutralizes the action of thrombin. Six naturally occurring antithrombins have been identified and are designated by Roman numerals I to VI. Of these, Antithrombin I (see FIBRIN) and ANTITHROMBIN III appear to be of major importance.Antithrombin III: A plasma alpha 2 glycoprotein that accounts for the major antithrombin activity of normal plasma and also inhibits several other enzymes. It is a member of the serpin superfamily.Antithrombin III Deficiency: An absence or reduced level of Antithrombin III leading to an increased risk for thrombosis.Antithrombins: Endogenous factors and drugs that directly inhibit the action of THROMBIN, usually by blocking its enzymatic activity. They are distinguished from INDIRECT THROMBIN INHIBITORS, such as HEPARIN, which act by enhancing the inhibitory effects of antithrombins.Heparin: A highly acidic mucopolysaccharide formed of equal parts of sulfated D-glucosamine and D-glucuronic acid with sulfaminic bridges. The molecular weight ranges from six to twenty thousand. Heparin occurs in and is obtained from liver, lung, mast cells, etc., of vertebrates. Its function is unknown, but it is used to prevent blood clotting in vivo and vitro, in the form of many different salts.Factor Xa: Activated form of factor X that participates in both the intrinsic and extrinsic pathways of blood coagulation. It catalyzes the conversion of prothrombin to thrombin in conjunction with other cofactors.Thrombin: An enzyme formed from PROTHROMBIN that converts FIBRINOGEN to FIBRIN.