MASP2, MBL associated serine protease 2, is a serum protease that plays an important role in the activation of the complement system via mannose-binding lectin (MBL). Binding of MBL to carbohydrate groups triggers MASP2 activation by auto-catalytic cleavage which leads to cleavage of C2 and C4, leading to their activation and to the formation of C3 convertase. MASP2 also binds to L-Ficolin and H-Ficolin, and when the complex binds pathogens the MASP is activated. Low MBL levels in serum are associated with increased susceptibility to infection, and an Asp105Gly mutation in MASP2 is associated with low MASP2 levels in serum. Reduced functional levels of MBL are also postulated to be associated with generation of autoantibodies in systemic lupus erythematosus, acting through a decreased clearance of apoptotic material. A truncated form of MASP2 that constitutes the amino 185 residues, Map19 is also a component of the MBL-MASP-1 complex.