Allergen immunotherapy, although effective, has been, to a degree, displaced by symptomatic therapy for bronchial asthma and allergic rhinitis. A major reason for its loss of popularity is the prolonged course of the treatment and the occurrence of reactions, both of which are due to the reaction of the allergenic extract with IgE. Thanks to recombinant technology, the possibility exists of modifying the major allergens so that they have reduced reactivity with IgE. Furthermore, the recombinant allergens can be combined with immunostimulatory sequences that favor the transformation from the Th2 phenotype characteristic of allergy to a Th1 cytokine profile. Thus, recombinant technology and new understanding of ways to alter the immune response to allergens promises to make allergy immunotherapy safer and more efficient.