Читать книгу Immunology - Richard Coico - Страница 88

The lectin pathway is initiated when MBL binds to the terminal polysaccharide residues of a pathogen such as a bacterium (see Figure 4.2B). MBL is structurally homologous to C1q in the classical pathway. Ficolin (not shown in the figure) has a structure similar to MBL and also binds carbohydrates, such as N‐acetylglucosamine or N‐acetylgalactosamine, on microbial surfaces.

Mannan‐binding lectin and ficolin are found in the circulation complexed with proteases, known as the mannose‐associated serine proteases (MASPs). Once bound to the bacterium, one of the proteases, MASP‐2, sequentially cleaves C4 and C2 to form C4b2a on the surface of the bacterium. As we discussed previously, C4b2a is also formed in the classical pathway; it is the C3 convertase that cleaves the next component in the pathway, C3. Thus, the lectin and classical pathways converge at this point.