Protective Capacity of Antibodies Against E. coli·Antigen with Special Reference to the Avidity

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Abstract
The antibodies against E. coli·antigen which developed in response to immunization of rabbits with formalin-killed bacteria were investigated for quantity, relative avidity, and protective capacity in experimentally infected mice. Since antibody quantitation by the ammonium sulphate precipitation (ASP) technique was not influenced by avidity, the ASP titres could be used to express the protective capacity of antisera per unit of antibody, ‘protective titre/ASP titre’. Using this ratio, low-avidity antibodies (Ko ≤ 0.25) were found less protective than antibodies of higher avidity. Increase of the avidity above Ko ≥ 0.45 did not further potentiate the protective activity of the antibodies. The commonly used indirect haemagglutination (IHA) technique and the recently developed enzyme-linked immunosorbent assay (ELISA) as applied in the present study were shown to be influenced by the antibody avidity. In the former, low avidity tends to give high antibody titres, whereas high avidity depresses the titres. In the latter technique, highly avid IgG antibodies seemed to give higher titres. The IHA titres correlated significantly to the protective titres only for antibodies of intermediate relative avidities (0.25 ≤ Ko ≤ 0.45), whereas the ELISA did not show any such correlation.

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