Two H-2-linked autosomal dominant immune response (Ir) genes Ir-IgG and Ir-IgA were demonstrated to be at separate loci. Ir-IgG controls the immune response to IgG (γ2a) myeloma proteins and Ir-IgA the immune response to IgA meyloma proteins. Both genes are associated with the H-2K region specificities of the H-2 chromosome, specifically Ir-IgG with H-2b and Ir-IgA with H-2a. Different recombinants derived from H-2a/H-2b crossovers were examined for their immune responsiveness to BALB/c IgG (γ2a) and IgA myeloma proteins. B10 (H-2b) parental type responded only to IgG; B10.A (H-2a) responded only to IgA. All the recombinants except for B10.A (4R) responded to either IgG or IgA. B10.A (4R), however, responded to both IgG and IgA. This indicated that the crossover event giving rise to B10.A (4R) occurred between the Ir-IgG and Ir-IgA loci.

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