Most important serine protease inhibitor in plasma that regulates the blood coagulation cascade. AT-III inhibits thrombin, matriptase-3/TMPRSS7, as well as factors IXa, Xa and XIa. Its inhibitory activity is greatly enhanced in the presence of heparin (By similarity). Forms protease inhibiting heterodimer with TMPRSS7. Plasma. Phosphorylated by FAM20C in the extracellular medium. Belongs to the serpin family.