Heparin is a polysaccharide which blocks the formation of blood clots (acts as an anticoagulant) by attaching to the protein antiprothrombin III, a protein which plays an important role in the formation of blood clots. The polysaccharide is composed of D-glucosamine-2,6-disulfate and D-glucuronic acid-2-sulfate. It is used to treat thrombosis and embolisms.