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[Immunology in the medical practice. XXVII. Mannose-binding lectin, an important link for nonspecific or hereditary immune reaction].

In the course of the past two decennia, a 3rd route of complement activation (next to the classical and the alternative routes) has been identified: the lectin route in which mannose-binding lectin (MBL) plays an essential role. MBL is produced in the liver. From the phylogenetic and functional points of view, complement activation via MBL falls in between the alternative and the classical routes and combines the advantages of the former (an early response, without the intervention of antibodies) with those of the latter (high specificity). The binding of MBL to the surface of a microorganism results in the activation of two serine proteases (MASP1 and MASP2) that are coupled to MBL. These enzymes can activate C4 and C2 so that, via the MBL route, the C3-convertase of the classical route (C4b2b) is produced long before there are any specific antibodies. The gene for MBL is located on the long arm of chromosome 10 and consists of a promoter gene and 4 exons coding for the protein. The prevalence of mutations in the MBL gene is about 10%, but in Africa South of the Sahara it is as high as 30%. MBL deficiency predisposes both children and adults to all sorts of infectious diseases, chronic diarrhoea, tonsillitis, otitis media, pneumonia, (meningococcal) meningitis, sepsis and osteomyelitis. Remarkably, MBL deficiency may actually be advantageous in some infections, because certain microorganisms use MBL or complement to invade the cell.