Hirudin

• potent polypeptide anticoagulant derived from the saliva of the blood-sucking leech Hirudo medicinalis
• use of this leech for medicinal purposes dates back to ancient Egypt
• leech application has been used in reconstructive microsurgery to alleviate venous congestion (pooling of blood in veins) in flaps and free pieces of tissue (read: partially torn or completely cut off fingers, ears, lips, nasal tips, etc.) that are being reattached, improving their survival
• inhibits thrombin, the central enzyme responsible for controlling the coagulation process (clotting of blood), by binding to it at a site separate from it's active site (the site where it does it's enzyme stuff) and then covering the active site over - is the most potent known natural thrombin inhibitor and one of the most potent natural anticoagulants
• first isolated and named in 1884, but it's structure and mechanism of action weren't discovered until 1955
• lepirudin and desirudin are recombinant (see this post) forms of hirudin produced in yeast cells (hirudin would be hideously expensive if it had to be harvested from leeches) that are used to prevent deep vein thrombosis in certain surgeries, but are less effective and more toxic compared to heparin when used to treat coronary heart problems
• since it is a peptide it must be given parenterally (with a needle into a body part - muscle, vein, beneath the skin, etc.)
• has served as a model for the design of new anticoagulant drugs, such as bivalirudin, a peptide that consists of a thrombin active site inhibitor linked via a short amino acid chain to a dodecapeptide (12 amino acids) analogue of a section of hirudin
  

Kikelj D. Peptidomimetic thrombin inhibitors. Pathophysiol Haemost Thromb. 2003 Sep-2004 Dec;33(5-6):487-91.

Whitaker IS et al. By what mechanism do leeches help to salvage ischaemic tissues? A review. Br J Oral Maxillofac Surg. 2005 Apr;43(2):155-60.