Competition between, and effectiveness of, IgG and IgM antibodies in indirect fluorescent antibody and other tests.

By using an indirect fluorescent antibody (IFA) procedure, we were recently able to demonstrate that a significant portion of natural antibody reactivity to Gram-negative bacteria resided in the IgG class as well as in the IgM class of antibodies (1). The magnitude of this IgG act ivi ty had apparently eluded detection in previous studies that used bactericidal, agglutination and other tests (2, 3). This suggested that the relatively greater sensitivity of these tests for IgM antibodies (4, 5) may have caused the substantial contribution of IgG antibodies to be overlooked. In order to examine this hypothesis, the reactivities of antibodies in purified 7S and 19S fractions of serum were studied in bactericidal, agglutination and IFA assays. The sensitivity to IgG antibodies of the IFA procedure and the relative insensitivity to IgG antibodies of bactericidal and agglutination tests were confirmed. In the course of these studies it was observed that IgM antibodies in some whole sera had less IFA reactivity at low dilutions than was present at higher dilutions. I t was found that this IgM prozone effect in the IFA assay could be explained by a competitive inhibition exerted by IgG antibodies. I t was further found that high concentrations of IgG antibodies partially inhibited the reactivity of IgM antibodies in the bactericidal reaction. These findings are also presented in this article.