Alpha-1-antichymotrypsin (AACT) is a plasma protease inhibitor and member of the serine protease inhibitor class (SERPIN). Alpha 1-Antichymotrypsin is an acute phase glycoprotein that functions by inhibiting proteases secreted by neutrophils and mast cells during periods of inflammation.  It is primarily produced by hepatocytes and locally expressed in histiocytes, including macrophages and Kupffer cells in the liver.  AACT polymorphism is tissue specific and influences protease targeting. Variation in AACT sequence has been implicated in Alzheimer's disease, and activity deficiency has been associated with liver disease. Mutations have been identified in patients with Parkinson disease and chronic obstructive pulmonary disease. Alpha 1-Antichymotrypsin is a useful marker for malignant fibrous histiocytomas to aid in differentiating from other soft-tissue tumors such as liposarcomas or Ewing's sarcoma.