Alpha-1-antichymotrypsin (AACT), also known as SERPIN family A member 3 (SERPINA3), is a glycoprotein primarily synthesized in the liver and astrocytes, then secreted into surrounding tissues or blood, where it plays significant roles. AACT is capable of binding to mast cell chymase and neutrophil cathepsin G, thereby protecting cells and tissues from damage. In various conditions such as fibromyalgia, multiple sclerosis, chronic heart failure, Alzheimer's disease, and certain cancers, AACT levels are elevated and can serve as a diagnostic or prognostic biomarker.