['Understanding the mechanisms by which the immune system induces and controls allergic inflammation at the T-cell epitope level is critical for the design of new allergy vaccine strategies.', 'We sought to characterize allergen-specific T-cell responses linked with allergy or peripheral tolerance and to determine how CD4(+) T-cell responses to individual allergen-derived epitopes change over allergen-specific immunotherapy.', 'Timothy grass pollen (TGP) allergy was used as a model for studying grass pollen allergies. The breadth, magnitude, epitope hierarchy, and phenotype of the DR04:01-restricted TGP-specific T-cell responses in 10 subjects with grass pollen allergy, 5 nonatopic subjects, and 6 allergy vaccine-treated subjects was determined by using an ex vivo peptide-MHC class II tetramer approach.', 'CD4(+) T cells in allergic subjects are directed to a broad range of TGP epitopes characterized by defined immunodominance hierarchy patterns and with distinct functional profiles that depend on the epitope recognized. Epitopes that are restricted specifically to either TH2 or TH1/TR1 responses were identified. Allergen-specific immunotherapy was associated with preferential deletion of allergen-specific TH2 cells and without a significant change in the frequency of TH1/TR1 cells.'] Does specific immunotherapy modify allergen-specific CD4 ( + ) T-cell responses in an epitope-dependent manner?