Alpha 1-antichymotrypsin is an alpha globulin glycoprotein that is a member of the serine proteinase inhibitor (serpin) family.It inhibits the activity of certain enzymes called proteinases, such as cathepsin G that is found in neutrophils, and chymases found in mast cells, by cleaving them into a different shape or conformation. This activity protects some tissues, such as the lower respiratory tract, from damage caused by proteolytic enzymes.

This enzyme is produced in the liver, and is an acute phase protein that is induced during inflammation. Deficiency of this protein has been associated with liver disease. Mutations have been identified in patients with Parkinson disease and chronic obstructive pulmonary disease.