Allergen Immunotherapy (AIT) is a specialized, long-term medical treatment for allergies that aims to change the underlying way the immune system reacts to harmless substances. Unlike traditional allergy medications, which only mask symptoms, AIT works to modify the body’s response to the allergen itself. The goal is the induction of sustained immune tolerance. This process allows the body to encounter the specific allergen in the future without triggering a disruptive allergic reaction, providing relief that can last even after the treatment course is finished.
The Science of Immune Tolerance
An allergic reaction begins when the immune system mistakenly identifies a harmless substance, like pollen or dust mites, as a threat. In this sensitized state, the immune response is dominated by a type of white blood cell called T helper 2 (Th2) cells. These Th2 cells promote the production of immunoglobulin E (IgE) antibodies, which bind to mast cells and basophils, priming them to release inflammatory chemicals like histamine upon re-exposure.
Allergen Immunotherapy introduces gradually increasing doses of the specific allergen, retraining the immune system through repeated, controlled exposure. This exposure promotes the development of regulatory T cells (Tregs), which are responsible for suppressing the allergic response. These Tregs secrete anti-inflammatory messengers, such as Interleukin-10 (IL-10), which help dampen the activity of the problematic Th2 cells.
The shift in the immune system’s focus also changes the type of antibodies produced. Instead of the allergy-causing IgE, the body starts producing allergen-specific immunoglobulin G subclass 4 (IgG4) antibodies. These IgG4 molecules are often called “blocking antibodies” because they intercept the allergen before it can bind to the IgE on mast cells. By preventing this initial binding, IgG4 effectively blocks the chain of events that leads to the release of histamine and the resulting allergy symptoms. This shift in the ratio of IgE to IgG4 is a measurable sign of successful desensitization and tolerance.
Delivery Methods and Treatment Phases
Allergen Immunotherapy is administered in two primary forms, each following a distinct schedule and route of delivery. Subcutaneous Immunotherapy (SCIT), commonly known as allergy shots, involves injecting a small dose of the allergen extract just under the skin. This method requires administration in a healthcare provider’s office due to the small risk of a systemic reaction.
Sublingual Immunotherapy (SLIT) is an alternative method where the allergen is given as a tablet or liquid drop placed under the tongue. SLIT can often be self-administered at home following an initial supervised dose in a clinic setting. While both SCIT and SLIT work on the same biological principle of desensitization, SCIT is generally considered the more potent option for a wider range of allergies.
Both delivery methods require the patient to complete two distinct treatment stages. The first is the build-up phase, which involves frequent administration, typically weekly for SCIT or daily for SLIT, with gradually increasing concentrations of the allergen. This phase continues until the patient reaches the maximum tolerated or effective dose, which usually takes about three to six months.
Once the maximum concentration is reached, the patient transitions into the maintenance phase. During this stage, the dose remains constant and the frequency decreases, usually to once a month for SCIT or daily for SLIT. To achieve long-lasting tolerance, the maintenance phase generally continues for three to five years.
Eligibility and Safety Considerations
Allergen Immunotherapy is typically recommended for individuals whose allergy symptoms are not adequately controlled by avoidance measures or standard medications like antihistamines and nasal steroids. Candidates must have specific allergies identified through testing, such as those to pollens, dust mites, or insect venom. The treatment requires a significant commitment.
Before starting, a physician will assess the patient’s overall health, as certain conditions can make the treatment unsafe. Uncontrolled or severe asthma is a contraindication because it increases the risk of a severe reaction to the allergen extract. Patients taking beta-blocker medications may be advised against AIT because these drugs can interfere with the effectiveness of epinephrine, the medication used to treat anaphylaxis.
The most common side effects are local reactions, such as redness, swelling, or itching at the injection site for SCIT, or oral itching for SLIT. These reactions are usually mild and manageable, often resolving within a few hours. The most serious safety concern is the rare risk of a severe, body-wide allergic reaction called anaphylaxis.
Because of the potential for anaphylaxis, SCIT must be administered in a clinic where medical staff can monitor the patient for at least 30 minutes following the injection. For SLIT, the first dose is typically given under supervision, and patients are often prescribed an epinephrine auto-injector for emergency use at home. Adherence to the prescribed dosing schedule is important for ensuring patient safety.