The influence of antibody on antibody formation to particulate antigen was examined in the rabbit with special reference to the importance of immunoglobulin type, the amount and relative proportion of antigen and antibody involved, and the specificity of this influence. 19S as well as 7S antibody was shown to be an effective inhibitor of antibody formation, although there was some evidence that 7S antibody was the more efficient of the two in doing so. The inhibitory effect of antibody was found to be specific for homologous antigenic determinants. Both 19S and 7S antibody were also able to enhance antibody formation. In contrast to the suppressive phenomenon, however, enhancement appeared to be nonspecific since antibody reactive with homologous (sheep red blood cell) determinants could enhance the response not only to homologous determinants but to heterologous (dinitrobenzene) determinants conjugated to the red blood cells as well. Smaller amounts of antibody were needed to enhance than to suppress antibody formation, and suppression and enhancement depended to some extent on the amount of antigen as well as to the amount of antibody used. The enhancing and suppressing influence of antibody on antibody formation appeared to be exerted concomitantly, for the response to some antigenic determinants was sometimes suppressed at the same time that the response to others was enhanced. It is suggested that enhancement or suppression of immunologic responses by antibody represents a different balance of at least two competing factors operating together: specific neutralization of appropriate determinants thus decreasing the total effective concentration of these determinants available to stimulate the formation of antibodies, and a nonspecific increase in the availability of antigen to immunologically competent cells.

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