Allergies are one of the most common ailments for which patients seek medical attention. They disrupt patients' lives and can even make others uncomfortable. Although symptomatic relief can be achieved with antihistamines and similar drugs, most patients prefer to live completely without symptoms and therefore turn to desensitization therapy. Here I will explain the mechanism underlying allergies and the different forms of desensitization therapies available today. An allergy is a hypersensitivity reaction of the immune system that occurs towards certain antigens to which the body perceives as a threat and to which it overreacts. Patients typically experience airway inflammation, among other symptoms caused by inappropriate histamine release. The purpose of desensitization therapy in the allergic patient is to induce tolerance to the allergens that cause the symptoms in the patient.1 There are several methods of desensitization therapy, the most important are subcutaneous injections and sublingual administration, and the less popular methods of therapy intralymphatic and transcutaneous immunotherapy.2 Before desensitization can begin, however, it is necessary to determine what the patient is actually allergic to. This is done by using purified antigen of the suspected allergen and performing a skin test or blood test. In the skin test, the suspected allergen is scratched into the skin and tests positive when a wheal develops.3 In the blood test, the patient's blood is mixed with the allergen to look for antibodies, particularly IgE. Therapy can only begin when the appropriate allergen is determined because immunotherapy is allergen specific. Allergens work by inducing a type 1... middle of paper... law of allergy, asthma and immunology. August 2006;97(2):126-137; quizzes 137-140, 202.12. Ohashi Y, Nakai Y, Murata K. Effect of fexofenadine pretreatment on the safety of immunotherapy in patients with allergic rhinitis. Annals of Allergy, Asthma, and Immunology: Official publication of the American College of Allergy, Asthma, & Immunology. April 2006;96(4):600-605.13. Larsen JN, Houghton CG, Vega ML, Lowenstein H. Production and standardization of allergenic extracts in Europe. Clinical allergy and immunology. 2008;21:283-301.14. Moingeon P, Mascarell L. Induction of tolerance through the sublingual route: mechanisms and applications. Clinical and developmental immunology. 2012;2012:623474.15. Iglesias-Cadarso A, Hernandez-Weigand P. Risk factors for systemic reactions to allergen immunotherapy. Current opinion in allergy and clinical immunology. December 2011;11(6):579-585.