C1 activation requires interaction with two separate Fc domains, so pentavalent IgM antibody is far more efficient at complement activation than IgG antibody (Muller-Eberhard and Kunkel 1961). Antibody binding results in a conformational change in the C1r component of the C1 complex and a proteolytic cleavage of C1r, activating it (Ziccardi and Cooper 1976). This reaction is irreversible under physiological conditions.
Cooper, NR, Ziccardi, RJ
serine-type endopeptidase activity of Antigen:IgG:C1Q:2xC1R:2xC1S [plasma membrane]
© 2023 Reactome