What is SLIT immunotherapy?

Sublingual allergen immunotherapy (SLIT) is a specialist allergy treatment designed to gradually train the immune system to tolerate a specific allergen. It may be used for selected patients with allergic rhinitis, allergic conjunctivitis and allergic asthma caused by triggers such as grass pollen, tree pollen, house dust mite or animal dander.

Unlike standard medicines, which mainly reduce symptoms while they are being taken, SLIT aims to reduce the body’s allergic response over time. Treatment is usually administered as liquid allergen drops under the tongue according to the prescribed treatment plan.

Why is earlier treatment being discussed?

A 2026 expert review in Current Opinion in Allergy and Clinical Immunology highlights a shift in thinking. Traditionally, allergen immunotherapy was often considered only after symptoms remained troublesome despite medication. However, newer real-world evidence suggests that starting immunotherapy earlier in suitable patients may offer a “window of opportunity” to influence the natural history of allergic airway disease.

This is particularly relevant for patients with persistent allergic rhinitis, with or without mild to moderate allergic asthma. Allergic rhinitis and asthma are closely linked, and untreated or poorly controlled nasal allergy may contribute to lower airway symptoms in some patients.

What does recent evidence show?

Large real-world studies, including the REACT programme and the EfficAPSI study, suggest that adding allergen immunotherapy to standard care may reduce medication use, severe asthma exacerbations and healthcare use over long-term follow-up.

The 2026 review also highlights that younger patients may gain particular benefit when treatment is started before allergic airway disease becomes more established. This supports the idea that allergic rhinitis should not always be viewed as a minor condition, especially when symptoms are persistent, seasonal year after year, or associated with wheeze, cough or exercise-related breathing symptoms.

Who may be suitable for SLIT immunotherapy?

SLIT is not suitable for everyone. It should only be considered after specialist allergy assessment, including a careful clinical history and confirmation that symptoms match relevant IgE sensitisation on skin prick testing or blood testing.

Why accurate diagnosis matters

The success of SLIT depends on selecting the correct allergen. For example, a patient with spring symptoms may be reacting to birch pollen, grass pollen, plane tree pollen, or more than one pollen. A patient with year-round symptoms may have house dust mite allergy, animal dander allergy, mould allergy, non-allergic rhinitis, or a combination of causes.

At a consultant-led allergy clinic, testing may include skin prick testing, specific IgE blood testing and, where appropriate, molecular allergy testing. This helps confirm whether immunotherapy is likely to be clinically relevant and which allergen should be prioritised.

How long does treatment take?

SLIT immunotherapy is usually a long-term commitment. Many treatment courses continue for around three years, depending on the allergen and clinical response. The aim is to achieve sustained immune tolerance and longer-lasting benefit, rather than short-term symptom relief only.

Can SLIT replace antihistamines and nasal sprays?

Not immediately. Symptomatic treatment remains important, especially during the early stages of immunotherapy and during high pollen exposure. Many patients continue to use nasal sprays, antihistamines or eye drops as needed. Over time, some patients may need less medication, but this should be reviewed individually.

Why choose a specialist allergy clinic?

SLIT should be prescribed and monitored by clinicians experienced in allergy diagnosis, asthma assessment and immunotherapy safety. A specialist clinic can help identify the most relevant allergen, assess asthma risk and monitor response throughout the course.

Summary

SLIT immunotherapy is increasingly recognised as a treatment that may modify the course of allergic respiratory disease. For suitable patients with confirmed allergic rhinitis, and especially those at risk of asthma progression, earlier discussion of immunotherapy may be clinically valuable.

The decision should always be personalised. The most important first step is accurate diagnosis, followed by careful selection of the correct allergen treatment plan.