Antibodies against Escherichia coli O antigen. Antibody amounts and avidities measured with ammonium sulfate precipitation technique.

The ammonium sulfate precipitation (ASP) technique, modified Farr assay, has been utilized to study antibodies to Escherichia coli O2, O4, O6 and O75 antigens the induction of which could not be ascribed to the effect of a defined antigen. It was noted that the antibody titers against the O6 antigen in children, 0.1-2.5 years of age, were of lower magnitude than the others, significantly for the O2 and O4 antigens. Comparison between boys and girls under and over 1.0 year of age revealed higher titers in sera from girls and in the older children. However, significant differences were obtained only in a few instances. The antibody avidities were in most cases low and no correlation was found between antibody quantity and avidity. In consecutive samples from adults a varying pattern of antibodies to the O6 and O75 antigens was observed.