SERPINA1 and alpha 1-antitrypsin deficiency: Alpha-1 antitrypsin deficiency (AATD) is a rare hereditary condition characterised by low circulating levels of the alpha-1 antitrypsin (AAT) protein, a serine protease inhibitor synthesised and secreted mainly by hepatocytes, that protects lung tissues from damage caused by proteolytic enzymes such as neutrophil elastase (NE).