Factor XII

Factor XII is synthesized in the liver and circulates as an inactive zymogen. Upon vascular injury exposing subendothelial collagen, or contact with artificial negatively charged surfaces (e.g., glass, kaolin used in lab PTT testing), Factor XII is activated by high molecular weight kininogen (HMWK) and prekallikrein to form Factor XIIa. Factor XIIa then activates Factor XI to XIa, propagating the intrinsic coagulation cascade toward the common pathway (activation of Factor X, thrombin generation, and fibrin formation). Factor XIIa also converts prekallikrein to kallikrein, which cleaves HMWK to release bradykinin, linking coagulation to the kinin-inflammatory system, and it can activate plasminogen to plasmin, tying into fibrinolysis. Despite its central role in vitro (measured by PTT), Factor XII deficiency does not cause clinically significant bleeding in vivo, because the tissue factor (extrinsic) pathway is sufficient for hemostasis in humans. Notably, some studies suggest Factor XII deficiency may be associated with an increased risk of thrombosis (e.g., historically noted in patient John Hageman, who died of a pulmonary embolism), possibly due to impaired fibrinolysis. This dissociation between prolonged PTT and lack of bleeding is a classic USMLE teaching point, often tested alongside Factor XI deficiency (mild bleeding) and Factor VIII/IX deficiency (hemophilia A/B, significant bleeding, prolonged PTT). Factor XII activation is also exploited therapeutically as a target for anticoagulants aiming to reduce thrombosis risk without increasing bleeding, since inhibiting the contact pathway does not impair normal hemostasis.