Allergen Immunotherapy (AIT) is the medical approach involving controlled exposure to an allergic trigger. This process is designed to modify the body’s reaction to substances like pollen, dust mites, or insect venom. AIT is the only treatment that seeks to fundamentally alter the underlying biological cause of the allergy, rather than simply managing symptoms. Understanding how this therapy works, the commitment it requires, and its documented success rates is important for anyone considering this long-term path to desensitization.
Understanding Allergen Immunotherapy
Allergen Immunotherapy (AIT) exposes the immune system to gradually increasing amounts of a specific allergen to build long-term tolerance. By repeatedly administering the substance, AIT essentially retrains the immune system to stop viewing that substance as a threat. This approach is fundamentally different from standard medications like antihistamines or nasal sprays, which only mask the symptoms of an allergic reaction.
The therapy is delivered in two primary forms: subcutaneous immunotherapy (SCIT) and sublingual immunotherapy (SLIT). SCIT, commonly known as allergy shots, involves injections given into the fatty tissue beneath the skin, usually in the upper arm. This method has been a proven treatment for over a century and is highly effective for a wide range of allergens.
SLIT, or sublingual immunotherapy, utilizes tablets or drops dissolved under the tongue. The U.S. Food and Drug Administration has approved SLIT for a limited number of allergens, including certain grasses, ragweed, and dust mites. While both forms induce immune tolerance, SCIT can treat multiple allergens simultaneously, whereas SLIT is typically limited to a single allergen per tablet.
How Gradual Exposure Retrains the Immune System
The success of AIT is rooted in its ability to shift the immune system’s response away from the traditional allergic pathway. An allergic reaction is driven by Immunoglobulin E (IgE) antibodies, which bind to mast cells and basophils, causing them to release inflammatory chemicals like histamine. AIT works by systematically redirecting this response.
Repeated, controlled exposure to the allergen stimulates the production of a different type of antibody, primarily Immunoglobulin G (IgG), and specifically the IgG4 subtype. These newly generated IgG antibodies act as “blocking antibodies,” intercepting the allergen before it can bind to the IgE on mast cells and trigger the allergic cascade. This mechanism effectively dampens the immediate allergic response.
The immune system’s re-education also involves the induction of regulatory T-cells (Tregs). These specialized cells maintain immune tolerance and are crucial for long-term success. Tregs secrete anti-inflammatory messengers, such as Interleukin-10 (IL-10), which suppress the activity of IgE-producing cells and Th2 cells that drive allergic inflammation. This dual action—blocking the allergen with IgG and calming the inflammatory response with Tregs—achieves lasting desensitization.
The Treatment Process and Patient Commitment
Committing to AIT requires a significant time investment and high patient adherence, as the therapy is a long-term undertaking. The treatment is divided into two distinct phases: the build-up phase and the maintenance phase. The initial build-up phase involves receiving injections with increasing concentrations of the allergen extract, typically once or twice a week.
This escalation continues until a target dose, known as the maintenance dose, is reached, which usually takes between three and seven months. Once the maintenance dose is achieved, the frequency of injections is reduced, usually to every two to four weeks. The maintenance phase is sustained for a period of three to five years, though some patients may require a longer duration to achieve lasting relief.
Side effects are common, with the most frequent being local reactions like redness, swelling, or itching at the injection site. A less common, but more serious, risk is a systemic reaction that can progress to anaphylaxis. For this reason, all SCIT injections must be administered in a medical facility. Patients are required to remain under observation for at least 30 minutes afterward to ensure a quick response to any severe reaction.
Documented Success Against Common Allergens
AIT is supported by strong clinical evidence, particularly for inhaled allergens and insect venom. For common seasonal and perennial allergies caused by triggers like grass pollen, tree pollen, dust mites, and animal dander, AIT significantly reduces symptoms and the need for allergy medications. AIT is the only treatment capable of modifying the natural progression of allergic disease, suggesting it can prevent the development of new allergies and reduce the risk of allergic rhinitis progressing to asthma.
The therapy is exceptionally effective for individuals with life-threatening allergies to Hymenoptera insect venom (e.g., from bees, wasps, and hornets). Venom Immunotherapy (VIT) has a documented success rate of over 90% in preventing future systemic allergic reactions in patients who have previously reacted to a sting. This effectiveness makes VIT a standard treatment for those at risk.
While AIT is highly successful for respiratory and venom allergies, its application to food allergies is limited. Traditional AIT is not used for food allergies, where strict avoidance remains the primary management strategy. However, specific emerging treatments like Oral Immunotherapy (OIT) for certain food allergies, such as peanuts, utilize a controlled exposure method approved to reduce the severity of allergic reactions.