heparin

an ANTICOAGULANT substance naturally present in the body and produced by liver and some white blood cells and in some other sites. It acts by inhibiting and neutralizing the action of the ENZYME thrombin (see BLOOD COAGULATION ) and is polysaccharide (carbohydrate) containing sulphur and amino groups. It is used medically to prevent blood coagulation in patients with THROMBOSIS and also in blood collected for sampling.