Mannan-binding protein (MBP) is a member of the collectin family, the serum proteins, bovine conglutinin and bovine collectin 43 (CL43), and two lung surfactant proteins (SP-A and SP-D). The collectins bind to terminal non-reducing sugar residues, mannose, GlcNAc, fucose and glucose. SP-D binds to maltose. They play important roles in innate immunity without involvement of antibodies.
The collectins are characterized by their unusual primary structures and also by their unique three-dimensional structures. MBP, conglutinin, and CL-43 are synthesized in liver and secreted into the circulation. SP-A and SP-D are mainly synthesized in the lung type II alveolar cells and Clara cells, and secreted into the alveolar space. The serum MBP content increases significantly after birth, and their levels vary greatly even among healthy adult individuals. The serum level increases after infection and remains elevated for longer periods compared with other typical acute phase reactants. The collectins have a consistent feature in the gene organization that the signal peptide, N-terminal segment, and the first few Gly-Xaa-Yaa triplets are encoded by a single exon. The rest of the collagen-like sequences are encoded by one (MBL and SP-A) to four (SP-D and conglutinin) exons. The 'neck' region and CRD are each encoded by a single exon.
All the known human collectin genes have been found clustered on chromosome 10q22-23. MBP can activate the complement system through the classical pathway (3). Upon binding to carbohydrate structures on the surface of microorganism, the lectin activates the Clr2Cls2 protease complex, like C1q (the overall structure of MBP is very similar to that of C1q), or a novel C1s-like protease, MBP-associated serine proteases (MASPs). These proteases, in turn, activate C2 and C4 of the classical complement pathway and bring about the killing and clearance of the targets. This MBP-mediated complement activation, called the lectin pathway, provides an additional mechanism of microorganism recognition by the complement system in the absence of specific antibodies. Three point mutations in the first exon of the human MBP gene result in MBP deficiency, which is associated with frequent infections in childhood and possibly also in adult.