F XII

Summary The plasma protein F XII (Hageman factor) has been shown to be linked with the plasma defence systems of coagulation, fibrinolysis, kallikrein-kinin and complement. It can be activated by surface contact activation and in solution. Surface contact activation is a complex phenomenon involving negatively charged surfaces, F XII, high molecular weight kininogen and plasma kallikrein. Fluid-phase activation can be effected by a variety of serine proteases. In both types of activation the F XII zymogen is converted to active enzymes. F XII levels in plasma are low or undetectable in both inherited deficiencies and in a variety of clinical conditions. F XII levels can also be elevated in some clinical conditions. Although discovered as a clotting protein F XII appears to play an important role in the kallikrein-kinin and fibrinolytic systems and also has effects on cells. Recent studies suggest that therapeutic blockade of activation of F XII can be of benefit in certain clinical conditions.