Alpha1-Antitrypsin (alpha1-AT), also known as alpha1-Proteinase inhibitor (alpha1-PI), is the most abundant protease inhibitor in blood and a member of the SERPIN family of proteinase inhibitors. Serum levels are typically 1.3 mg/ml (2.5 micrometers) but alpha1-AT is an acute phase protein and concentrations can rise four-fold during inflammatory episodes or tissue injury. Low levels in circulation have been associated with pulmonary disease such as emphysema. alpha1-AT is a single chain molecule with a mass of 52,000 daltons that is produced primarily in the liver and to a lesser extent by blood monocytes and intestinal epithelium. Based on association rates, the primary target enzyme for alpha1-AT is believed to be neutrophil elastase, but alpha1-AT is a broad-spectrum inhibitor formany serine proteinases and the main role of alpha1-AT in vivo is likely that of a "backup" inhibitor and proteinase scavenger in fluids and tissues. Although the association rates of alpha1-AT with other enzymes are lower, the high concentration in plasma makes it an important inhibitor of activated Protein C, activated FXI, thrombin and plasmin. Enzyme inhibition by alpha1-AT occurs through proteolytic cleavage between Met³⁵⁸ and Ser³⁵⁹, which induces a conformational change in alpha1-AT locking the enzyme into a stable, inactive 1:1 enzyme-inhibitor complex.