Allergy desensitization, formally called allergen immunotherapy, works by gradually retraining your immune system to stop overreacting to harmless substances like pollen, dust mites, or pet dander. It does this by exposing you to slowly increasing doses of the allergen over months and years, which shifts the type of immune response your body mounts. About three out of four patients see meaningful symptom improvement within the first year, and the benefits can persist for years after treatment ends.
What Happens Inside Your Immune System
When you have allergies, your immune system has essentially made a mistake. It treats a harmless protein (say, a grass pollen grain) like a dangerous invader, producing large amounts of a specific antibody called IgE. These IgE antibodies sit on the surface of immune cells throughout your nose, eyes, and airways. The next time you encounter that allergen, the IgE antibodies grab onto it, triggering those cells to dump histamine and other inflammatory chemicals. That’s your sneezing, itching, and congestion.
Desensitization rewires this response at several levels. The most important change involves a special population of immune cells called regulatory T cells. These are essentially peacekeepers. As your body receives repeated, controlled doses of the allergen, it begins producing more of these regulatory cells, which release chemical signals that calm the inflammatory cascade instead of amplifying it. Think of it as your immune system gradually learning that the allergen isn’t worth fighting.
At the same time, your body starts producing a different type of antibody called IgG4. These act as blocking antibodies. They intercept the allergen before it can bind to the IgE antibodies sitting on your inflammatory cells. With the allergen intercepted, the cells never get the signal to release histamine. The net effect is a fundamental shift: your immune system moves from an aggressive, allergic-type response to a tolerant one. This isn’t just symptom suppression like taking an antihistamine. It’s a structural change in how your immune system categorizes the allergen.
Shots vs. Tablets: Two Ways to Get There
There are two main delivery methods, and both produce similar results through the same underlying mechanism.
Subcutaneous immunotherapy (allergy shots) involves injections given in a doctor’s office. The treatment starts with a buildup phase lasting 6 to 10 months, during which you receive injections of increasing allergen doses, typically once or twice a week. Once you reach your target dose, you enter a maintenance phase where you get a shot every few weeks for 3 to 5 years.
Sublingual immunotherapy (under-the-tongue tablets or drops) involves placing a tablet containing the allergen extract under your tongue daily. The first dose is given in a medical setting for observation, but after that you take it at home. The convenience is a major advantage. Sublingual immunotherapy has similar effectiveness to shots but causes fewer bodywide reactions. Most side effects are limited to itching or tingling in the mouth.
Shots carry a small risk of systemic allergic reactions, which is why they’re always given in a clinical setting with a waiting period afterward. The rate of significant reactions is roughly 0.2% per injection. Serious anaphylaxis is rare, but about 5 to 7% of patients experience some type of reaction over the course of treatment.
How Long Treatment Takes and How Long It Lasts
The full course of immunotherapy is a commitment. After the initial buildup period, maintenance treatment continues for 3 to 5 years total. Cutting the course short matters: patients who stop before completing three years are more likely to see their symptoms return within a year. Those who complete at least three years of treatment typically maintain their improvement for 2 to 3 years after stopping, and some retain benefits even longer.
In one study of grass pollen sublingual immunotherapy, symptom scores remained 25 to 36% lower than placebo across five consecutive pollen seasons, including two full years after treatment had ended. This lasting effect is what separates immunotherapy from every other allergy treatment. Antihistamines and nasal sprays only work while you’re taking them. Desensitization changes the underlying disease.
What Improvement Actually Looks Like
About 36% of patients with allergic rhinitis notice improvement within the first six months. By one year, the majority are experiencing real relief. In clinical practice surveys, after more than a year of treatment, over 90% of rhinitis patients reported improvement in runny nose, and roughly 87% reported less nasal congestion. Sneezing improved in 88%, and even eye-related symptoms like itching and redness improved in 76 to 80% of patients.
The medication reductions are equally striking. Before starting immunotherapy, about 90% of rhinitis patients were taking antihistamines. After treatment, that dropped to 49%. Nasal steroid spray use fell from 83% to 36%. For asthma patients, the results followed a similar pattern: inhaled steroid prescriptions dropped from 80% to 49%, and rescue inhaler use fell from 61% to 38%.
Who Is a Good Candidate
Immunotherapy isn’t the first step in allergy treatment. It’s recommended when you’ve already tried standard medications and allergen avoidance but your symptoms still interfere with daily life or sleep. The European Academy of Allergy and Clinical Immunology guidelines specify that candidates should have moderate to severe symptoms confirmed by allergy testing (a skin prick test or a blood test showing allergen-specific IgE antibodies). You need a clear link between your symptoms and specific allergens for the treatment to be targeted effectively.
It can also be considered for milder cases when someone wants to take advantage of the long-term, disease-modifying effects. This is especially relevant for children with hay fever, where grass pollen immunotherapy may reduce the risk of developing asthma later on. People who have had a serious allergic reaction to insect stings (bee or wasp venom) are also strong candidates, with venom immunotherapy following a faster 10-week buildup schedule.
Food Allergy Desensitization
Oral immunotherapy for food allergies uses the same core principle (gradually increasing exposure to build tolerance) but works differently in practice and carries higher risks. Instead of shots or tablets for environmental allergens, patients swallow tiny, measured amounts of the food protein, with doses increased every few weeks under medical supervision.
The results can be dramatic. In peanut allergy trials, 84% of treated children were able to tolerate 20 peanuts by the end of the protocol, compared to almost none in the placebo group. One landmark trial showed a 25-fold increase in the amount of peanut protein patients could safely consume. For egg allergy, desensitization rates reached 75% after about two years of treatment. In cow’s milk studies, patients went from reacting to just 40 milligrams of milk protein to tolerating over 5,000 milligrams.
The trade-off is safety. Most patients undergoing food oral immunotherapy experience some allergic reactions during treatment. In the peanut trial that led to the first FDA-approved therapy, over 95% of participants aged 4 to 17 had adverse events, though most were mild or moderate. Anaphylaxis occurred in about 14% of the treatment group, compared to 3% in the placebo group. About 12% of treated patients withdrew from the trial due to side effects. For cow’s milk immunotherapy, roughly 1 in 11 patients needed an epinephrine injection at some point during treatment.
Food oral immunotherapy is best understood as raising the threshold for a dangerous reaction rather than creating a true cure. A child who previously could go into anaphylaxis from a trace amount of peanut might, after treatment, safely tolerate the equivalent of several peanuts. That margin of safety can be life-changing for families managing food allergies, even if complete tolerance isn’t always achieved.