Complement activation in relation to protective function of antibodies to Escherichia coli O antigens

The relation between complement activation in an indirect hemolysis assay of E. coli O antibodies and their protective function in experimentally infected mice was analyzed. The antibody amounts and avidities were estimated in antisera and in IgG and IgM antibody fractions by the ammonium sulphate precipitation (ASP) technique of Farr. The protection obtained was due to both IgG and IgM antibodies and showed a close relation to the quantity of antibodies measured by the ASP technique. No link between complement activation capacity of the antibodies and their protective function could be seen. In the system used, the IgM antibodies showed a correlation between antibody avidity and complement activating efficiency, whereas the IgG antibodies activated complement poorly. The results do not indicate a major function of the complement in antibody-mediated protection against E. coli in this experimental system in mice.