Enhancement of the IgG antibody production to Escherichia coli O antigen by prior exposure to serologically different E. coli bacteria.

Exposure of rabbits and mice to serologically different bacteria enhanced the synthesis of IgG but often suppressed the formation of IgM O antibodies in response to subsequent immunization with O6 bacteria. The effect differed with the O antigen used. A higher enhancing effect on the IgG antibody synthesis was associated with less suppression of the IgM-antibody formation. The antibody avidity of both immunoglobulin classes was relatively low compared to that in the ordinary immune response to O6. The findings indicated an enhancing action of endotoxin on the IgG antibody response to the O6 antigen.