Allergen Immunotherapy
On this page
Direct answer
Allergen immunotherapy is the repeated administration of gradually increasing doses of a causative allergen to induce lasting tolerance, given as subcutaneous injections or sublingual tablets for about three to five years. It is indicated for allergic rhinitis, allergic asthma with proven sensitisation and hymenoptera venom anaphylaxis. Uncontrolled asthma and beta-blocker use are the main cautions, because they make a severe systemic reaction harder to treat.
What you must remember
- Mechanism: Repeated exposure shifts the response from Th2 towards regulatory T cells and IL-10, with IgG4 blocking antibodies rising, which explains why benefit can outlast the course.
- Duration: Standard courses run about three to five years, whichever route is used, and this separates immunotherapy from symptomatic antihistamine therapy.
- Indications: Allergic rhinitis and allergic asthma with relevant sensitisation, and hymenoptera venom allergy after a systemic sting reaction. Food allergy is not a standard indication.
- Subcutaneous route: Frequent build-up injections of increasing concentration, then maintenance injections every four to six weeks, with observation in the clinic for about 30 minutes after each dose.
- Sublingual route: Daily self-administered tablets or drops, with a lower systemic reaction risk, though not zero, so the first dose is commonly given under supervision.
- Contraindications: Uncontrolled or severe asthma, beta-blocker therapy and unstable cardiovascular disease are key cautions, and starting during pregnancy is not recommended.
- Safety rule: Epinephrine must be immediately available, and a dose is withheld or reduced when the patient is unwell, has poor asthma control or has missed doses.
A supervised subcutaneous course, step by step
A 28-year-old with perennial rhinitis and house dust mite sensitisation on skin prick testing is a typical candidate for a supervised subcutaneous course. The plan rests on proving the sensitisation and screening for the conditions that make a systemic reaction dangerous.
- Confirm the indication: Match the symptom history to sensitisation on skin prick testing or specific IgE, and confirm that symptoms persist despite pharmacotherapy.
- Screen for contraindications: Review asthma control, cardiovascular status and beta-blocker use, and defer the course while asthma is unstable or an infection is active.
- Build up the dose: Give frequent injections, typically weekly, of increasing concentration until the target maintenance dose is reached over a few months.
- Maintain the dose: Continue injections every four to six weeks, observing each patient for about 30 minutes with epinephrine at hand.
- Adjust after missed doses: After a delay, restart at a reduced dose according to the interval missed, rather than resuming the previous concentration.
- Review and set the duration: Assess symptom scores and rescue medication use at intervals, and plan a total course of about three to five years.
Where the exam tries to catch you
Most errors here involve patient selection and the safety rules rather than the mechanism.
- Trap: Starting standard immunotherapy for food allergy because the child is sensitised. Correct: Standard allergen immunotherapy is not indicated for food allergy, and oral immunotherapy is a separate, specialist-run approach.
- Trap: Giving an injection during an asthma flare because the dose is due. Correct: Uncontrolled asthma is a key contraindication, since a systemic reaction is harder to treat in an asthmatic patient, so the dose is deferred until control returns.
- Trap: Continuing a beta-blocker and assuming epinephrine will work normally. Correct: Beta-blockers can blunt the response to epinephrine and make anaphylaxis harder to reverse, so the drug is a major caution.
- Trap: Treating sublingual immunotherapy as entirely risk-free. Correct: Sublingual tablets carry a lower systemic reaction risk but not zero, so monitoring and the safety rules still apply.
Frequently asked questions
Which allergen immunotherapy route carries the lower risk of systemic reactions?
Sublingual allergen immunotherapy carries a lower systemic reaction risk than subcutaneous allergen immunotherapy, though the risk is not zero. Subcutaneous immunotherapy needs clinic-based injections and observation after each dose, which is why it is given under closer supervision.
What is the standard duration of allergen immunotherapy?
Allergen immunotherapy is usually continued for about three to five years, whichever route is used. Benefit can persist after the course stops, which distinguishes it from symptomatic drugs such as antihistamines.
Which patients are poor candidates for allergen immunotherapy?
Patients with uncontrolled or severe asthma, those taking beta-blockers and those with unstable cardiovascular disease are poor candidates for allergen immunotherapy. Starting it during pregnancy is also not recommended.
Why must epinephrine be available during subcutaneous allergen immunotherapy?
Systemic reactions, including anaphylaxis, can occur during or after a subcutaneous dose of allergen immunotherapy. Epinephrine must be immediately available and the patient observed for about 30 minutes after each injection.
Is venom immunotherapy indicated after a bee sting that caused only a large local reaction?
Venom immunotherapy is generally indicated after a systemic reaction to a hymenoptera sting, not after a large local reaction alone. The decision also depends on the severity of the prior reaction and the patient's risk profile.