What is SLIT - and how does it work?

Professor Adam Fox, Co-Founder AllergyRhino

When people talk about immunotherapy for allergies, one of the most accessible and widely used forms today is SLIT, short for sublingual immunotherapy.

But what exactly is it? How does it “retrain” the immune system? And who is a good candidate? Let’s break it down.

What Does SLIT Mean?

Sublingual = “under the tongue.”

Immunotherapy = treatment designed to change the immune system’s response to an allergen, not just suppress symptoms.

With SLIT, small, controlled doses of the

allergen extract (for example, grass pollen, house dust mite, or tree pollen) are placed under the tongue — in tablet or drop form. The allergen is held there briefly, then swallowed.

AllergyRhino co-founder, Professor Adam Fox explains it simply:

“If you have hay fever caused by grass pollen, the immunotherapy tablet we’ll use is made from grass pollen itself. Those high doses retrain your immune system so it’s less ‘interested’ in grass pollen, so when the season hits, you get fewer symptoms and rely less on medication.”

Because SLIT is usually self-administered (after the first dose under clinical supervision), it’s much more convenient than injection immunotherapy.

Why SLIT? The Key Advantages

SLIT offers several important benefits that make it increasingly popular across Europe, Australia, and now the UK:

• Excellent safety profile: Severe reactions are extremely rare. “It’s dramatically safer than the older injection immunotherapy,” says Dr Fox. “Severe allergic reactions are extremely rare — most people only notice mild itchiness in the mouth that settles quickly.”

• Home use: After the first supervised dose, ongoing treatment can be taken at home. No weekly clinic visits or injections.

• Long-term impact: SLIT can lead to lasting improvement even after treatment stops, and evidence shows it may reduce the risk of developing new allergies or asthma later in life.

• Proven efficacy: Large studies consistently show SLIT reduces allergy symptoms and the need for medication.

  
  

That said, it’s not a “quick fix.” As Professor Fox explains:

“Immunotherapy is preventative, not instant. You start a few months before pollen season and see gradual improvement, often 30–40% better in the first year.”

The Mechanism: How SLIT Re-Educates the Immune System

When the allergen extract sits under your tongue, specialised immune cells in the lining of the mouth begin a process of “retraining.” Over time, your immune system becomes more tolerant and less reactive.

Here’s what happens step-by-step:

1. Local exposure: Dendritic cells in the mouth process the allergen in a tolerant environment.

2. Immune recalibration: Your body increases “regulatory” immune cells (Tregs, Bregs) that calm the allergic response.

3. Antibody shift: IgE (the allergy antibody) decreases, while IgG4 (“blocking antibody”) increases — reducing inflammation.

4. Less reactivity: Tissues like the nose and eyes become less sensitive.

5. Lasting tolerance: Even after stopping treatment, the immune changes often persist.

“Immunotherapy is the only treatment that actually gets to the root cause of an allergy and changes your underlying immune response.” - Professor Adam Fox

Evidence & Clinical Considerations

Effectiveness

Clinical studies and real-world experiences show that SLIT significantly improves symptom control and reduces reliance on medication, especially for grass pollen and dust mite allergies.

“At the end of a three-year course, the benefits last for many years afterwards — that’s in stark contrast to regular hay fever medication, which only works while you’re taking it,” says Dr Fox.

Safety

Local side effects like mild tingling or itching under the tongue are common but usually settle within minutes. Severe reactions are extremely rare (around 0.02% in large studies).

“For a small proportion of people, the first week can be uncomfortable,” explains Dr Fox. “We might suggest antihistamines at the start, and after that, most people tolerate it perfectly well.”

Who Should Consider SLIT?

SLIT is most suitable for people who:

• Have a confirmed IgE-mediated allergy to grass, tree pollen, or dust mites

• Struggle with symptoms despite medication

• Prefer not to rely on daily antihistamines or nasal sprays

• Are aged 5 or older

• Have controlled asthma (uncontrolled asthma is a contraindication)

  
  

“We see two main groups benefit: those whose hay fever really affects daily life despite medication, and those who don’t want to depend on nasal steroids or tablets forever.” - Professor Adam Fox

Real-World Experiences

For many people, SLIT marks a turning point after years of frustration.

One of our customers, a long-time hay fever sufferer, described her journey through the AllergyRhino community:

“I’ve tried everything, antihistamines, sprays, even acupuncture. They’d work for a bit and then stop. I needed something that went deeper than just masking symptoms.

After starting SLIT, I noticed the difference even by the first summer. I could go for a jog in the park without sneezing five times in a row. It’s not gone completely yet, but it’s a big relief.”

Another customer, who struggled with dust allergies for decades, shared a similar feeling of freedom:

“It’s not going to kill me, but it’s so annoying. I used to come home from the office exhausted, nose running, eyes sore, it wiped me out. Since starting SLIT, that constant cycle’s broken. I can work a full day and still have energy left.”

These experiences mirror what we see clinically, SLIT doesn’t erase allergies overnight, but it reshapes the allergy journey. It gives people back control.

How We at AllergyRhino View SLIT

At AllergyRhino, we see SLIT as a cornerstone of long-term allergy management, especially for those who:

• Have already optimised avoidance and medication

• Want to treat the cause, not just the symptoms

• Are ready for a structured, multi-year approach that builds lasting tolerance

Our team supports every patient through:

• Confirming the right allergen match

• Timing treatment correctly (e.g., starting grass SLIT by January)

• Monitoring progress and comfort

• Reviewing whether to continue or adjust treatment

“If you’re going to start, it’s about doing it properly, right allergen, right timing, and sticking with it long enough to get lasting benefit.” - Professor Adam Fox

Summary & Next Steps

SLIT (sublingual immunotherapy) is a safe, evidence-based way to retrain the immune system to tolerate allergens instead of overreacting to them.

It’s a preventative, long-term therapy that helps patients move from constant symptom management to real immune change — restoring tolerance from the inside out.

At AllergyRhino, we’re committed to helping people make that transition with the right knowledge, medical guidance, and support every step of the way.

If you’re ready to move beyond managing your allergy to changing it. SLIT could be the key step.