Hirudin is a naturally occurring blood anticoagulant present in salivary glands of leeches. It is peptide in nature. Full length hirudin is made up of 65 amino acids. The coagulation of blood mainly occurs due to the activity of serine protease enzyme thrombin which converts fibrinogen into fibrin to form blood clots. The principal inhibitor of thrombin in normal blood circulation is antithrombin III. Hirudin has the similar activity to antithrombin III, which inhibit the procoagulant activity of thrombin.