Based upon studies of the relationships of blocking and binding antibodies to IF in patients with pernicious anemia, it has been hypothesized that the antigenic site on human IF for the blocking antibody may be a more potent immunogen than the corresponding site for binding antibody. The results of this study are in agreement with this hypothesis in that rabbits given a single injection of approximately 270 ng human IF were found to have low concentrations of blocking antibody but no detectable binding antibody in their serum.Rabbits challenged with the same amount of IF complexed to B12 produced both blocking and binding antibodies. The concentration of blocking antibody in the latter animals was significantly higher than in the group immunized with IF, indicating that B12 enhances the immunogenic potencies of both antigenic sites on IF.Binding antibody was detected in both the γM and γG classes of immunoglobulins. Blocking antibody was found to be predominantly γG.
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