Allergy immunotherapy: Stimulation of the immune system with gradually increasing doses of the substances to which a person is allergic. The aim is to modify or stop the allergy by reducing the strength of the IgE response. This form of treatment is very effective for allergies to pollen, mites, animal dander, and especially stinging insects. Allergy immunotherapy usually takes six months to a year to become effective, and injections are usually required for three to five years.