Alpha-1 antitrypsin or α1-antitrypsin （A1AT, α1AT, A1A, or AAT） is a protein belonging to the serpin superfamily. As a type of enzyme inhibitor, it protects tissues from enzymes of inflammatory cells, especially neutrophil elastase, and has a reference range in blood of 0.9–2.3 g/L, but the concentration can rise manyfold upon acute inflammation. When the blood contains inadequate amounts of A1AT or functionally defective A1AT （such as in alpha-1 antitrypsin deficiency）, neutrophil elastase is excessively free to break down elastin, degrading the elasticity of the lungs, which results in respiratory complications. A1PI also acts to induce locomotion of lymphocytes through tissue including immature T cells through the thymus where immature T cells mature to become immunocompetent T cells that are released into tissue to elevate immune responsiveness.