Copyright © 2016, OceanSide Publications, Inc.Background: Allergen immunotherapy (AIT) leads to the production of antiallergen immunoglobulin (IgG) or "blocking antibody" in the serum and an increase in antiallergen IgG and IgA in nasal secretions. There is also a decrease in the usual rise in antiallergen IgE that occurs after the pollen season. Methods: In this paper, mechanisms of action of allergen immunotherapy is reviewed. Results: Regulatory T (Treg) cells and their cytokines, primarily interleukin (IL) 10 and transforming growth factor beta, suppress T-helper type 2 immune responses and control allergic diseases in many ways. AIT induces a shift in the proportion of IL-4-secreting T-helper type 2 cells in favor of IL-10-secreting inducible Treg cells specific for the same allergenic epitope that increases in number and function. Different types of inducible Treg control several facets of allergic inflammation. There are two main types of immunotherapy: subcutaneous immunotherapy and sublingual immunotherapy. Subcutaneous immunotherapy is efficacious and is indicated for the reduction of seasonal symptoms. Sublingual immunotherapy involves the regular self-administration and retention of allergen extract under the tongue for 1-2 minutes before the extract is swallowed. The allergens cross the mucosa in 15-30 minutes and are then captured by tolerogenic dendritic cells and processed as small peptides. Next, via the lymphatic system, a systemic immune response is created to produce an early decrease in mast cell and basophil degranulation. Conclusion: AIT is indicated for the treatment of moderate-to-severe intermittent or persistent symptoms of allergic rhinitis. AIT can be administered to those >5 years of age and has been shown to be safe in children as young as 3 years of age. In this article, AIT and other types of immunotherapies were discussed as well as the indications for immunotherapy.