Mannan-binding lectin (MBL)-associated serine proteases (MASPs) circulate in plasma as zymogens in complexes with MBL and with L- and H-ficolin. Upon binding of MBL or ficolin to pathogen-associated molecular patterns, the MASPs are activated. MASP-2 can now cleave C4 and C2 to generate the C3 convertase, C4bC2b. The functions of the other two MASPs, MASP-1 and MASP-3 have not been elucidated. MASP-1 can cleave C2, and with low efficiency also C3, and may serve a function through direct C3 activation. No natural substrate for MASP-3 has been identified. MBL deficiency, occurring at a frequency of about 10%, is the most common congenital immunodeficiency and is associated with susceptibility to infections and autoimmune disorders. Inherited MASP-2 deficiency has been described as the result of a mutation causing the exchange of aspartic acid with a glycine at position 105, a position in the first domain, CUB1, involved in calcium binding. This mutation abolishes the binding to MBL and ficolins, and deprives MASP-2 of functional activity. The index case suffered from recurrent severe infections and autoimmune reactions. The gene frequency of the mutation among Caucasians is 3.6%. It is not found in Chinese, who present a different mutation also associated with MASP-2 deficiency.