MBL is a serum collectin that can bind to mannose residues on microbial cell walls. Upon binding of MBL to pathogens, two serine proteases found in serum (MBL-lectin associated serine proteases 1 and 2) become activated. These serine proteases in turn activate the classical pathway at C2 and C4. MBL deficiency may arise because of one of three point mutations in gene encoding for this protein. Polymorphysms in the promoter region of the gene also influence serum levels of MBL. Low serum levels of MBL may result in an increased incidence of pyogenic infections in young children below two years of age. MBL deficiency is also overrepresented in patients with pneumococcal or meningococcal sepsis. However, the clinical significance of this finding has been disputed. This is because large, long-term prospective studies of adults in Scandinavia have failed to identify increased morbidity due to infectious diseases in MBL-deficient individuals. Therefore, except in young children who have not yet acquired a wide repertoire of protective antibodies, MBL deficiencies on its own does not appear to be a significant risk factor for bacterial sepsis.