Specific allergen immunotherapy is the administration of increasing amounts of specific allergens to which the patient has type I immediate hypersensitivity. It is a disease modifying therapy, indicated for the treatment of allergic rhinitis, allergic asthma, and hymenoptera hypersensitivity. Specific IgE antibodies for appropriate allergens for immunotherapy must be documented. Indications for allergen immunotherapy include (1) inadequate symptom control despite pharmacotherapy and avoidance measures, (2) a desire to reduce the morbidity from allergic rhinitis and/or asthma or reduce the risk of anaphylaxis from a future insect sting, (3) when the patient experiences undesirable side effects from pharmacotherapy, and (4) when avoidance is not possible. Furthermore, patients may seek to benefit from economic savings of allergen immunotherapy compared with pharmacotherapy over time. Several studies have reported that immunotherapy in children with allergic rhinitis appears to prevent the development of new allergic sensitizations and/or new-onset asthma. Humoral, cellular, and tissue level changes occur with allergen immunotherapy including large increases in antiallergen IgG(4) antibodies, a decrease in the postseasonal rise of antiallergen IgE antibodies, reduced numbers of nasal mucosal mast cells and eosinophils, induction of Treg cells, and suppression of Th2 more than Th1 lymphocytes. There is a corresponding increase in IL-10 and transforming growth factor beta. In the United States, allergen immunotherapy is administered by the subcutaneous route in the physician's office, whereas primarily in some countries in Europe, it is administered for allergic rhinitis and asthma by the sublingual route by the patient at home.