The protein encoded by this gene is highly expressed in late-pregnancy serum and is similar in structure to alpha-2-macroglobulin. The encoded protein, which acts as a homotetramer, inhibits the activity of all four classes of proteinases. This protein contains cleavage sites for several proteinases. Upon binding of a proteinase, the conformation of this protein changes to trap the proteinase, limiting its activity. This protein appears to be elevated in the sera of presymptomatic Alzheimer's disease patients. [provided by RefSeq, Dec 2016].