English: The two arms of the contact system involve factor XII (FXII), prekallikrein (PK) and high molecular weight kiningogen (HK). FXII and PK are proteases and HK is a non-enzymatic co-factor. In both arms FXII binds to a surface and reciprocal activation of FXII and PK occurs, forming FXIIa and PKa. PK cleaves HK to form cHK, and a peptide is released, known as bradykinin. BK and its derivatives bind to bradykinin receptors B1 and B2 to mediate inflammation. Additionally, PK can activate factor XI (FXI), which can initiate the coagulation cascade.
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