Allergy immunotherapy is based on the administration of increasing amounts of the disease-eliciting allergens in order to yield allergen-specific non-responsiveness. Success of this therapy is associated with modulation of the immune response to allergenic molecules at the level of T-helper cells and the induction of blocking antibodies. The extracts used for immunotherapy are highly heterogenous preparations from natural sources and contain additional components, mostly proteins which are not well defined. Recombinant DNA technology offers novel tools for production of pure and well-characterised allergens for specific immunotherapy. However, high IgE reactivity of pure recombinant allergens is associated with an increased risk of potentially life-threatening anaphylactic reactions. A major improvement in allergen-specific immunotherapy may be achieved by using genetically engineered recombinant allergens with reduced anaphylactic activity. Recently the site- directed mutagenesis technique has been applied successfully to produce variants of major grass, birch and oilseed rape allergens with reduced IgE reactivity but retained T-cell reactivity. These modified allergens with reduced anaphylactic potential are novel candidates for safer and more effective allergen-specific immunotherapy.