The present invention is drawn to an immunogen derived from a protein allergen, which is a) a non-anaphylactic immunogenic recombinant fragment of the protein allergen which contains an IgG epitope partly but not wholly overlapping an IgE epitope of the protein allergen; b) a polymeric form of the fragment, in which the fragment constitutes the monomeric units; or c) a non-anaphylactic recombinant polymeric form of the protein allergen having 2 10 monomeric units, in which the protein allergen constitutes the monomeric units. The present invention is further drawn to the use of the immunogen for in vitro diagnoses of type I allergy and hyposensitization.