SERPINA1 and alpha 1-antitrypsin deficiency: Alpha-1 antitrypsin deficiency (AATD) is a rare inherited disease characterized by decreased plasma levels of alpha-1 antitrypsin (AAT), a serine protease inhibitor glycoprotein synthesized and secreted mainly by hepatocytes (and to a lesser extent by monocytes, macrophages, and pulmonary epithelial cells).