Mannan-binding lectin pathway
The Mannan-binding lectin pathway is homologous to the classical complement pathway. This pathway uses a protein similar to C1q of the classical complement pathway, which binds to mannose residues and other sugars in a pattern that allows binding on multiple pathogens. Mannan-binding lectin (MBL; also called mannose-binding lectin) is a protein belonging to the collectin family that is produced by the liver and can initiate the complement cascade by binding to pathogen surfaces.
MBL is a 2-6 headed molecule that forms a complex with MASP-I (Mannan-binding lectin Associated Serine Protease) and MASP-II, two protease zymogens. MASP-I and MASP-II are very similar to C1r and C1s molecules of the classical complement pathway and are thought to have a common evolutionary ancestor. When the carbohydrate-recognising heads of MBL bind to specifically arranged mannose residues on the phospholipid bilayer of a pathogen, MASP-I and MASP-II are activated to cleave complement components C4 and C2 into C4a, C4b, C2a, and C2b. C4b and C2b combine on the surface of the pathogen to form C3 convertase (C4b2b), while C4a and C2a act as chemoattractants.
It has been found that people deficient in MBL experience a substantial increase in infections during the early years of childhood.