α1-antitrypsin, an acute phase protein, is the prototypic member of the serpin super family and a major inhibitor of serine proteases. As an acute phase protein, α1-antitrypsin is thought to play an important role in limiting host tissue injury at sites of inflammation.

A1AT is a 52-kDa serpin (serine protease inhibitor), and, in medicine, it is considered the most prominent serpin; the terms α1-antitrypsin and protease inhibitor (P_i) are often used interchangeably.Like all serine protease inhibitors, A1AT has a characteristic secondary structure of beta sheets and alpha helices. Mutations in these areas can lead to non-functional proteins that can polymerise and accumulate in the liver (infantile hepatic cirrhosis).