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Expert Reviews in Molecular Medicine: http://www.expertreviews.org/
Accession information: Vol. 5; 23 May 2003 Citation
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Activation and regulation of the complement system
Karen Francis, Johan van Beek, Cecile Canova, Jim W. Neal and Philippe Gasque
Figure 3. Activation and regulation of the complement system. Complement is a highly conserved innate immune cascade of 30 or so proteins that interact to recognise and kill pathogens. Activation is triggered by one of three pathways – classical, alternative and lectin – depending on the nature of the foreign molecule and therefore the activating surface. The classical pathway is activated primarily by the interaction of C1q with immune complexes of antibody with antigen, but can also be achieved after interaction of C1q with nonimmune molecules. The alternative pathway does not depend upon the presence of immune complexes and leads to the deposition of C3 fragments on the target cells. The lectin pathway shares several molecules with the classical pathway and is activated by binding of MBL to carbohydrates expressed on pathogens but not generally found on ‘self’ cells. The end result of all three pathways is either the opsonisation or the destruction (through formation of the lytic molecule C5b-9) of nonself cells and target organisms. The system is regulated by proteins such as C1-INH, C4bp, fI, fH, DAF, CR1, MCP, CD59, S protein and clusterin, which help to protect the host from immune attack. Some of these inhibitors are soluble (in bold, italics) and some are membrane associated (in bold, boxed). Abbreviations: C, complement component; C1-INH, C1 inhibitor; C4bp, C4b-binding protein; DAF, decay accelerating factor; f, factor; MASP, MBL-associated serine proteinase; MBL, mannan-binding lectin; MCP, membrane cofactor protein (fig003pgc).
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