Mannose-binding lectin (MBL), also called mannose-binding protein (MBP), is structurally similar to C1q. It binds multiple bacterial monosaccharides (mannose, N-acetylglucosamine, N-acetylmannosamine, and L-fucose) and activates associated serine proteases (MASP-2, MASP-1), which in turn activate the classical pathway. MBL deficiency is inherited in an autosomal co-dominant manner, with heterozygotes having about 10% of the normal functional level of MBL and homozygotes having less than 1% functional levels of MBL. Mannose-binding lectin / MBL deficiency has been reported in 2-7% of the British population.
Children with low levels of MBL are at increased risk of infection, especially after the loss of passive immunity (acquired from maternal IgG) but before the formation of protective antibody (around 6 months of age). Pediatric MBL-deficient individuals are especially at increased risk for meningococcal disease. Adult MBL deficiency has been associated with susceptibility to many bacterial infections but particularly encapsulated bacteria. MBL deficiency has also been associated with an increased susceptibility and clinical severity of fungal, protozoal, and viral infections as well as community-acquired pneumonia and Neisseria meningitis.
It has been proposed that dysfunction of the MBL pathway, like dysfunction of the classical pathway, also leads to autoimmune diseases. MBL deficiency has been associated with systemic lupus erythematosus / SLE across several ethnic backgrounds, and in one study SLE was increased twofold. MBL deficiency in cystic fibrosis patients leads to earlier Pseudomonas infection, an increased rate of decline in lung function, and earlier death from end-stage lung disease. In common variable immunodeficiency patients (CVID) with MBL deficiency, there is an increase in bronchiectasis, pulmonary fibrosis, and respiratory insufficiency.
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