KRN T cells can recognize two self MHC alleles with differing biological consequences. They respond to the foreign peptide RN(42-56) bound to I-Ak or alternatively initiate autoimmune arthritis by interacting with a self Ag, GPI(282-294), on I-Ag7. Five surface amino acid differences between the two MHC molecules collectively alter which peptide side chains are recognized by the KRN TCR. In this study, it is shown that mutation of only two of these residues, α65 and β78, in I-Ak to their I-Ag7 counterparts is sufficient to allow recognition of the TCR contacts from GPI(282-294). To provide a detailed mechanism for the specificity change, the distinct contributions of each of these two mutations to the global effect on peptide specificity were analyzed. The α65 mutation is shown to broaden the spectrum of amino acids permissible at P8 of the peptide. In contrast, the β78 mutation alone blocks KRN TCR interaction with I-Ak and requires the simultaneous presence of the α65 mutation to preserve recognition. In the presence of the α65 mutation, the β78 residue broadens peptide recognition at P3 and prevents recognition of the P8 L in RN(42-56), thus producing the observed specificity shift. These results localize the functionally relevant differences between the surfaces of two self-restricted MHC molecules to two residues that have counterbalanced positive and negative contributions to interaction with a single TCR. They highlight how subtle structural distinctions attributable to single amino acids can stand at the interface between foreign Ag responsiveness and pathogenic autoreactivity.