Coagulation inhibition. Molecular model of a complex (left) of heparin (yellow), antithrombin (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. Thrombin converts fibrinogen (light blue, top right) into fibrin, which forms a mesh that traps blood cells. Activation of antithrombin and its binding to thrombin prevents the conversion of fibrinogen and so has an anticoagulant effect.