Nonatopic subjects immunized with short ragweed developed both reaginic IgE and blocking IgG antibodies to the antigen. Administration of 250 µg-specific ragweed IgG antibodies 1 week before, simultaneously, or up to 2 weeks after start of immunization with ragweed extract abrogated the production of anti-ragweed reaginic antibodies which was measured by skin reactivity. Formation of specific ragweed IgG antibodies was unaffected by IgG treatment. These results indicate selective regulation of an immune response in humans by IgG.

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