alpha 1 Antitrypsin is a plasma protein synthesised in the liver. It consists of a single polypeptide chain and has a molecular weight of 52 kDa. AAT (Alpha-1-antitrypsin) is an inhibitor of serine proteases. Although AAT's primary target is elastase, the enzyme has also a moderate affinity for plasmin and thrombin. It irreversibly inhibits trypsin, chymotrypsin and plasminogen activator. The aberrant form inhibits insulin-induced NO synthesis in platelets, decreases coagulation time and has proteolytic activity against insulin and plasmin. Short peptide from AAT is reversible chymotrypsin inhibitor. It also inhibits elastase, but not trypsin. Its major physiological function is the protection of the lower respiratory tract against proteolytic destruction by human leukocyte elastase (HLE). AAT deficiency is related to emphysema and liver disease.