Hageman factor

Factor XII (Hageman factor) is synthesized in the liver and circulates as an inactive zymogen. When blood contacts a negatively charged surface (such as exposed subendothelial collagen, glass in laboratory testing, or activated platelets releasing polyphosphates), Factor XII undergoes a conformational change and becomes activated to Factor XIIa. This is the initiating step of the intrinsic pathway of the coagulation cascade, measured by the partial thromboplastin time (PTT) laboratory test. Factor XIIa activates Factor XI to XIa, propagating the coagulation cascade toward thrombin generation and fibrin clot formation. Beyond coagulation, Factor XIIa has important 'cross-talk' functions: it activates prekallikrein to kallikrein, which cleaves high-molecular-weight kininogen (HMWK) to release bradykinin, a potent vasodilator and mediator of inflammation and pain. This kinin pathway is clinically important in hereditary angioedema (though that condition is primarily due to C1 esterase inhibitor deficiency, C1-INH also regulates Factor XIIa and kallikrein, so its deficiency leads to unregulated bradykinin production and Factor XII activation contributes to attacks). Factor XIIa also plays a role in fibrinolysis by activating plasminogen indirectly and interacts with the complement system. Clinically, Factor XII deficiency is notable because it prolongs the PTT significantly on laboratory testing, but affected individuals do NOT have a bleeding tendency - this is because the primary in vivo initiator of coagulation is the tissue factor (extrinsic) pathway via Factor VIIa, not the contact pathway in most physiological settings. In fact, some studies suggest Factor XII deficiency may be associated with slightly increased thrombotic risk, though this remains an area of ongoing research. This dissociation between prolonged PTT and lack of clinical bleeding is a classic USMLE teaching point, contrasting with Factor VIII, IX, or XI deficiency (hemophilias), which DO cause bleeding disorders despite also prolonging PTT. Historical note: named after John Hageman, the patient in whom the deficiency was first identified in 1955.