The present invention relates to compositions for preventing or treating allergy to ragweed by tolerisation. The compositions are based on combinations of peptide fragments derived from the major allergen in ragweed pollen, Amb a 1. The invention also relates to products, vectors and formulations which may be used to provide polypeptides of the invention in combination. The invention further relates to in vitro methods for determining whether T cells recognize a polypeptide of the invention, and for determining whether an individual has or is at risk of a condition characterized by allergic symptoms in response to a ragweed allergen.
