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MASP-2 (mannose/mannan binding lectin (MBL) associated serine protease-2) is a serum protein predominantly synthesized by the liver as a ~75kDa protein and is one of the key molecules of the innate immune system. It is mainly bound to multimeric protein complexes, such as MBL, the three ficolins (M-ficolin, L-ficolin and H-ficolin) and collectin kidney 1 (CL-K1, alias CL-11). These complexes serve as pathogen receptors, which are further bound to MASP-1, a serine protease. Binding of these complexes to their appropriate pathogenic ligands auto-activates MASP-1. Active MASP-1 in turn acts on its substrate, MASP-2, and thereby activates it. In a cascade of proteolytic cleavage events, MASP-2 activates complement proteins C4 and C2 to form C4b2a (classical C3 convertase), thereby converging the lectin pathway with the classical pathway of complement activation. Further, MASP-2 activity is regulated by several factors, including the serine protease inhibitor C1INH and by interaction with other proteins of the lectin complement pathway.

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