The immune responsiveness to human and bovine α-lactalbumin (HuALA and BoALA) was found to be under the control of immune response (Ir) gene(s) linked to the major histocompatibility complex. H-2k mice responded to both HuALA and BoALA, whereas H-2d, s, and f mice respond only to HuALA; H-2b mice were nonresponders to both HuALA and BoALA. A survey with B10.A recombinant mouse strains .enabled us to map the Ir gene in the I-A subregion. The responsiveness was shown to be dominant in F1 mice. The coimmunization of BoALA and HuALA resulted in the suppressed secondary antibody response to HuALA in B10.S (H-2S) and BALB/c (H-2d) but not in C3H (H-2k) suggesting that the low responsiveness against HuALA in these strains is due to an active suppression. The transfer of splenic T cells of B10.S mice primed with BoALA into syngeneic animals suppressed the response to HuALA. T cells specific for a particular epitope present on BoALA appeared to suppress the immune response to other epitopes on HuALA. Thus, the presence of epitope-specific suppressor T cells seems to account for this Ir-gene-controlled low responsiveness to ALA in H-2smice
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