Abstract: The invention relates to a method of desensitising a patient to a polypeptide allergen the method comprising administering to the patient a peptide derived from the allergen wherein restriction to a MHC Class II molecule possessed by the patient can be demonstrated by the peptide and the peptide is able to induce a late phase response in an individual who possesses the said MHC Class II molecule. A composition comprising a plurality of peptides derived from a polypeptide allergen wherein for at least one of the peptides in the composition restriction to a MHC Class II molecule can be demonstrated, and the composition is able to induce a late phase response in an individual possessing the given MHC Class II moleculeis also provided. The invention also relates to a method of selecting a peptide for use as an immunotherapeutic agent for desensitising a patient to a polypeptide allergen capable of eliciting an allergic response in the patient.