The present invention notably concerns a novel major allergen from ragweed pollen, named Amb a X, as well as isoallergens and isoforms thereof. Fragments of the aforementioned polypeptides and homologous polypeptides, in particular homologous polypeptides in related plant species, also make part of the invention. The invention also concerns uses of said polypeptides, in particular for diagnosing and preventing or treating an allergy.