Factor XIa (Last Updated - 2/28/2005)

-makes up a step of the intrinsic pathway
-formed via activation of Factor XI --> this reaction is catalyzed by Factors XIIa & IIa (thrombin)
-Factor XIa (along with tissue factor (Factor III) & Factor VIIa) in turn catalyzes the activation of Factor IX to IXa

-click here to view an image of the clotting cascade

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References:

Goodman & Gilman's: The Pharmacologic Basis Of Therapeutics - 10th Edition - 2001. Chapter 55.