The structural basis of the T cell response against immunodominant tetanus toxin (TT)-derived peptides was investigated using TT-specific T cell clones raised from a DRB1*0301 homozygous donor. Three peptides forming T cell epitopes were identified, including one, TT(1272–1284), that stimulated four different TT-specific T cell clones. TCR sequence analysis revealed that these synonymous TCR shared only arginine at the third position of the CDR3β loop. This prominent residue may form a salt bridge with a corresponding aspartate at the relative position 8 (P8) of the antigenic peptide TT(1272–1284) as suggested from amino acid replacement analysis. A similar scenario was observed for a second TT epitope, TT(279–296), and its corresponding TCR. These examples show that immunodominance may result from a single strong amino acid interaction between TCR CDR3β loops here in contact with the C-terminus of the antigenic peptide. Such a dominant interaction could compensate for weaker contacts between other residues of the TCR and the antigenic peptide, and would allow the recognition of a single peptide–MHC complex by a broader synonymous TCR repertoire and could thus contribute to its immunodominance.