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Mannose/mannan-binding lectin

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Mannose/mannan-binding lectin (MBL) is a serum lectin synthesized (as a ~32 kDa peptide) by the liver and is one of the key molecules of the innate immune system. Each peptide has an N (amino)-terminal cysteine-rich region, a middle stretch of a collagen-like sequence, and a carbohydrate recognition domain (CRD) in the C (carboxy)-terminus. Three identical peptides form a structural subunit, similar to a collagenous triple helix, which is the basic building block of all circulating molecular forms of MBL. Further oligomerization of these structural subunits by disulphide bonds in the N-terminal region results in MBL molecules of different sizes (from dimers to hexamers), but the hexameric form is probably the most common. MBL-associated serine proteases (MASPs) bind to MBL multimeric forms to stabilize the molecule. MBL is a pattern-recognition receptor and the CRDs of MBL serve to bind to a wide range of pathogens such as bacteria, viruses and protozoa, by recognizing carbohydrate moieties on their surfaces. There are two pathways by which MBL can participate in a host defense response: 1) MBL activates the lectin complement pathway via MASPs, that converges with the classical complement pathway, at the level of complement C4 (C4-A or C4-B), and 2) MBL may also act directly as an opsonin, enhancing phagocytosis by binding to cell-surface receptors present on phagocytic cells.

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