MBL has an oligomeric structure (400-700 kDa), built of subunits that contain three identical peptide chains of 32 kDa each.
Although MBL can form several oligomeric forms, there are indications that dimers and trimers are not biologically active and at least a tetramer form is needed for activation of complement.
The complement system can be activated through three pathways the classical pathway, the alternative pathway, and the mannose-binding (MB) lectin pathway. The most-recently discovered mannose-binding lectin pathway activates complement through the mannose-binding lectin protein. MBL binds to carbohydrates found on the surface of many pathogens.
MBL in the blood is complexed with (bound to) another protein, a serine protease called MASP-2 (MBL-associated serine protease).
In order to activate the complement system when MBL binds to its target (for example, mannose on the surface of a bacterium), the MASP protein functions to cleave the blood protein C4 into C4a and C4b. The C4b fragments can then bind to the surface of the bacterium, and initiate the formation of a C3 convertase.
The subsequent complement cascade catalyzed by C3 convertase results in creating a membrane attack complex, which causes lysis of the pathogen that MBL bound to.
It is produced in the liver as a response to infection, and is part of many other factors termed acute phase proteins.
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Ji X, Gewurz H, Spear GT (2005). "Mannose binding lectin (MBL) and HIV.". Mol. Immunol.42 (2): 145-52. doi:10.1016/j.molimm.2004.06.015. PMID 15488604.