Role in Disease
Factor XII deficiency is rare disorder that is inherited in an autosomal recessive manner. Unlike other clotting factor deficiencies, factor XII deficiency is totally asymptomatic and does not cause excess bleeding. Mice lacking the gene for factor XII, however, are less susceptible to thrombosis. The protein seems to be involved in the later stages of clot formation rather than the first occlusion of damages in the blood vessel wall.
Factor XII does play an important role in clot formation during in vitro measurements of the partial thromboplastin time, which causes these measurements to be markedly prolonged in patients with factor XII deficiency, usually well beyond even what is seen in hemophilia A, hemophilia B, or factor 11 deficiency. As a result, the main concern related to factor XII deficiency is the unnecessary testing, delay in care, worry, etc. that may be prompted by the abnormal lab result. All of this, including the mechanism of inheritance, also holds true for the other contact factors, prekallikrein (Fletcher factor) and high molecular weight kininogen.
Normal or excess levels of factor XII can predispose towards greater risk of venous thrombosis due to factor XII's role as one of the catalysts for conversion of plasminogen to its active fibrinolytic form of plasmin.
Hageman factor is also activated by Endotoxin, especially Lipid A.
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