Molecular model of a complex of heparin (yellow), antithrombin (light blue) and thrombin (dark blue). Heparin is a polysaccharide (sugar) molecule found naturally in the human body that acts as an anticoagulant (prevents the formation of blood clots). Binding of heparin to antithrombin activates it. Antithrombin is an inhibitor of coagulation proteases, such as thrombin, that promote the formation of blood clots. Therefore activation of antithrombin and its binding to thrombin has an anticoagulant effect.