Heparin's principal anticoagulant action is binding to antithrombin III. Antithrombin III is a natural anticoagulant protein in the body that inhibits the activity of several blood clotting factors, including factors IIa, IXa, and Xa. By binding to antithrombin III, heparin enhances its inhibitory effect on these clotting factors, preventing the formation of blood clots.