We have examined the isotypic pattern of the response of mice to the Ir gene-controlled antigen, dinitrophenyl-ovomucoid (DNP-OM). H-2k mice are high responders (HR); H-2b,d mice are low responders (LR). The isotype patterns of HR and LR strains differ both quantitatively and qualitatively. In the primary response to doses of 20-100 Mg DNP-OM, HR strains produce IgM and IgG antibodies, whereas LR strains produce only IgM. Background genes modify the kinetics of the IgGl primary response in HR strains, but no background was found which allowed an IgG response in a LR strain. In secondary responses, priming with 0.2 μg DNP-OM increases secondary responses in HR strains, and decreases them in LR strains. Control of this response maps to I-A, and is not altered by the bm12 I-Ab mutation. The LR phenotype is dominant in (HR × LR)F1 mice.
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