Alpha-1-antitrypsin (A1AT) is a glycoprotein of MW 54 kDa with a pI of 4.8. It is a serine protease inhibitor that acts to limit the amount of cell damage and death at the tissue site. Different isoenzymes exist which together form the Protease inhibitor system. Phenotype M is the most common and certain variants (S and Z) are associated with A1AT deficiency which can lead to emphysema and liver disease.