Factor XII (F XII, Hageman factor) is a 80kD, single chain glycoprotein that circulates in blood as an inactive zymogen. F XII plays an important role in blood coagulation, fibrinolysis, and kinin generation. When plasma comes into contact with negatively charged surfaces such as glass, kaolin, or dextran sulfate, F XII is bound to the surface and undergoes limited proteolysis by plasma kallikrein, resulting in two active enzyme forms, ɑ-F XIIa and β-F XIIa. ɑ-F XIIa consisting of a 28kD light chain containing the protease domain, and a 52kD heavy chain containing the anionic surface-binding domain. β-F XIIa consists of two disulfide-linked chains, a 2 kD light chain and a 28 kD heavy chain containing the catalytic domain. Defects in Factor XII do not cause any clinical symptoms and the sole effect is that whole-blood clotting time is prolonged.