The role of factor XI in coagulation: A matter of revision

In 1991 it was demonstrated that, besides factor XII, thrombin is capable o f activating factor XI in vitro. Thrombin-dependent activation of factor XI is an integral part of the revised theoretical model of coagulation in whi ch coagulation is initiated by the extrinsic pathway and maintained by thro mbin-induced activation of clotting factors V, VIII, and XI, In this review , special interest is given to the new role of factor XI in coagulation, wi th emphasise on data supporting the concept of thrombin-mediated factor XI activation in vivo. Furthermore, activation of factor XI in human disease, especially atherosclerotic disease, measured by newly developed immunologic assays, is discussed. The relation of factor XI to fibrinolysis through ac tivation of the carboxypeptidase, thrombin-activatable fibrinolysis inhibit or (TAFI) by thrombin provides an explanation for the bleeding tendency obs erved in factor XI-deficient patients, The probable link with factor XI-med iated TAFI activation may have clinical and therapeutic consequences and de serves further study.