Monoclonal or polyclonal IgM-anti-SRBC antibodies were used to enhance the anti-SRBC PFC response in mice. For potentiation to occur, the IgM antibodies must always be presented with the antigen for which they have specificity. No enhancement of anti-SRBC response above control levels was noted with either antibodies alone or with antibodies used together with non-cross-reacting antigens.

The degree of enhancement was independent of whether only one or several different monoclonal IgM antibodies were used. Likewise, the fine specificity variation among the antibody clones failed to influence the anti- SRBC potentiation, which was shown to vary only with the amount of IgM bound to SRBC measured by hemolytic titers.

The response against epitopes on the SRBC other than those the IgM recognized was also enhanced. This was determined by injecting SRBC and a monoclonal anti-SRBC IgM that did not crossreact with GRBC into mice, and measuring the response against both antigens. Normally SRBC and GRBC cross- react at the B cell level to approximately 30 percent, and in this experiment they did so both in the control group and in the IgM group.

Using antigens that only cross-react significantly at the T cell level (SRBC and HRBC), IgM-antibodies would only enhance the anti-HRBC response if SRBC and HRBC were inoculated together. No anti-HRBC potentiation was noted when antibodies were injected alone or together with either SRBC or HRBC.

The data indicate that the constant part of the IgM molecule is of major importance in determining its enhancing properties in acute IgM-mediated potentiation of the immune responses. No evidence was obtained for a decisive role of variable regions. Furthermore, no general B cell activating properties of either mono- or polyclonal IgM-anti-SRBC antibodies could be demonstrated.

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