Abstract
The complement cascade consists of 18 plasma proteins which upon activation regulate the inflammatory response, effect bacteriolysis, solubilize immune complexes, and opsonize microorganisms. Activation of the complement cascade may occur via the antibody-dependent classical pathway (C1, C2, and C4) or the antibody-independent alternative pathway (factor B and factor D) (Arnaout 1985; Pangburn 1983; Reid 1981). Activation of early acting components of both pathways occurs by limited proteolysis. Complexing of a single dimer of IgG or IgM with antigen on cell surfaces activates a single C1 molecule (Borsos 1965). Two subcomponents of C1, C1r and CIs, are sequentially activated, and CIs activates C4 and C2 by limited proteolytic cleavage. An unstable enzyme complex is formed between cleavage fragments of C4 and C2, C4b2a, which activates C3 by cleavage of a 9-kD fragment from the alpha chain of C3. A C3-converting enzyme which cleaves C3 at the same site can also be generated via the alternative pathway (Pangburn 1983). Factor B is a peptide structurally and functionally similar to C2. It is cleaved after complexing with C3b by factor D, a serine protease analogous to CIs. The alternative pathway C3 convertase (C3bBb) is a complex of the 70-kD activation product of factor B, Bb, and C3b. Because the larger activation product of C3, C3b, is part of this complex, a positive amplification loop is established. Activation of C3 by either pathway results in sequential activation of the terminal components C5-C9 and in deposition of C3b, the major activation product of C3, on cell surfaces.
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References
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© 1987 Springer-Verlag Berlin Heidelberg
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Cole, F.S. (1987). Complement Function in the Neonate. In: Burgio, G.R., Hanson, L.Å., Ugazio, A.G. (eds) Immunology of the Neonate. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-71094-0_8
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