Because in the industrialized world allergies afflict a high percentage of the population approaches to treatment are necessary that besides alleviating the symptoms also modify the immune reaction and curb the progression of allergies. Such therapeutic and preventive effect is a feature of immunotherapy. It is performed by repeated and controlled applications of the allergen in low doses without the risk for systemic reactions but with satisfactory therapeutic efficacy. Immunotherapy changes the immune reaction to tolerance to allergen on all the levels of the allergic response from the primary response of antigen presenting cells, B and T lymphocytes to the final inflammatory response of mastocytes and basophiles by reducing their sensitivity. Clinical studies confirm both the short-term efficacy associated with seasonal alleviation of symptoms and long-term efficacy based on immune changes many years after the end of therapy. Classical immnotherapy uses allergenic extracts but they are increasingly replaced by recombinant allergens and hypoallergens beacuse of low efficacy and risk of adverse reactions in treatment with extracts. Recombinant approaches with genetic engineering facilitate the reduction of allergenicity and preservation of immunogenicity of molecules. Alleregnicity can be reduced by chemical modification, allergen polymerization, fragmentation or recomabinant fusion with conformational distortion of IgE binding epitopes.