Serine protease inhibitor. The major targets of this inhibitor are plasmin and trypsin, but it also inactivates matriptase-3/TMPRSS7 and chymotrypsin (By similarity). Forms protease inhibiting heterodimer with TMPRSS7. Expressed by the liver and secreted in plasma. Proteolytically cleaved at Pro-31 by both the prolyl endopeptidase FAP form and antiplasmin-cleaving enzyme FAP soluble form to generate mature alpha-2-antiplasmin. Belongs to the serpin family.