Most of what you eat every day is, in biological terms, a foreign substance entering your body. Food proteins are molecules your immune system has never evolved to recognise as "self". They arrive in enormous quantities through your gut. In a healthy person, the immune system encounters them and does nothing. That is not an accident. It is the result of an active process called oral tolerance, and understanding it is the final piece of this series.
What oral tolerance is
Oral tolerance is your immune system's ability to recognise food proteins that come in through your mouth and digestive system as safe, and to actively hold back any response against them.
This is not passivity. It is something your immune system does on purpose. When food proteins arrive in your gut, specialised immune cells in the gut lining sample them and carry information about them to the rest of the immune system. In someone with good oral tolerance, this triggers regulatory T-cells — a type of white blood cell whose job is to put the brakes on immune responses rather than fire them up. The regulatory T-cells signal that this protein is safe. The immune system stands down [1].
Food allergy, from this angle, is a failure of oral tolerance. Your immune system encountered a food protein — peanut, milk, egg — and instead of producing a regulatory response, it produced an inflammatory one. The IgE-producing pathway switched on. Mast cells were armed. The conditions for anaphylaxis were set [1].
The window that matters most
Oral tolerance is not built at a single point in time and then locked in. It is established gradually during a critical period — mainly in early infancy — when the gut immune system is actively calibrating itself based on the environment it encounters.
During this window, exposure to diverse food proteins, alongside a healthy and developing gut microbiome, promotes tolerance across a wide range of foods. The timing matters. A young infant's gut immune system is at its most receptive to building tolerogenic responses — responses that accept rather than attack. The same exposure offered later may not work as well [2].
This is the biology behind what the LEAP trial showed, described in Part 2 of this series. Early, regular peanut consumption in high-risk infants cut peanut allergy by 80% by age five — because it engaged the oral tolerance mechanism at the right moment. Avoidance denied that training, and allergy developed instead [3].
What disrupts oral tolerance
Several things can interfere with how oral tolerance develops, and understanding them helps explain why you may have the condition you have.
The gut microbiome is one of the most important factors. The regulatory T-cell responses that underpin oral tolerance depend partly on signals from gut bacteria, particularly bacteria that produce compounds called short-chain fatty acids. A gut microbiome with reduced diversity produces fewer of these protective signals. Children with lower gut microbiome diversity in their first months of life have been found to have higher rates of food allergy [4].
The gut lining itself is another factor. The lining of the small intestine is only one cell thick in places. When it is disrupted — by inflammation, infection, or other things — proteins can enter your body in ways that trigger immune attack rather than tolerance [4].
Skin sensitisation is a third route researchers have identified. When a child with eczema — which weakens the skin barrier — is exposed to a food protein like peanut through damaged skin rather than through eating, the immune environment around that exposure is inflammatory rather than tolerogenic. The immune system learns to attack the protein rather than accept it. This helps explain why avoiding dietary peanut in eczema-prone infants seemed sensible at the time but actually increased sensitisation through skin contact [3].
If you developed your food allergy in childhood, one or more of these factors may have played a role. That is not something you could have controlled or prevented. But understanding it gives you a more complete picture of why your immune system does what it does.
What some people with food allergy are already doing about it
The science of oral tolerance has not only changed how we think about preventing food allergy. It has opened up a way to treat established allergies.
Oral immunotherapy, or OIT, is a treatment in which a person with confirmed food allergy gradually eats increasing amounts of their allergen under medical supervision. Starting with a tiny dose and building up slowly over weeks and months, the aim is to shift the immune system toward a more regulated state — not necessarily getting rid of the allergy entirely, but raising the threshold at which a reaction happens, so that accidental exposure carries less risk. A systematic review of randomised controlled trials found OIT to be effective at achieving desensitisation for peanut, cow's milk, and hen's egg allergy, though side effects are common and the treatment needs careful management [5].
OIT programmes for peanut, milk, and egg are offered through specialist allergy clinics in the UK and internationally. Palforzia, a pharmaceutical peanut OIT product previously approved by the US Food and Drug Administration (FDA) and the European Medicines Agency (EMA), is being withdrawn by its manufacturer, with hospitals advised not to start new patients after April 2026. This does not mean OIT itself is ending. It means the landscape is changing. The Natasha Allergy Research Foundation is funding a major trial using everyday foods rather than pharmaceutical products for OIT, which could make the treatment far more widely available and affordable if the results are positive.
If you are interested in OIT, the starting point is a conversation with your allergist or specialist allergy clinic. The availability of specific programmes changes, so up-to-date clinical guidance is essential. OIT is not suitable for everyone, and it requires a significant commitment of time and careful monitoring. But for some people, it represents a meaningful shift in how they live with their allergy.
What this means for you
Across these four posts, we have gone from the evidence that food allergy is genuinely rising, through the biological reasons behind that rise, to coeliac disease and finally to oral tolerance — the mechanism that decides whether your immune system accepts or attacks the food you eat.
In each case, the picture is the same. Your condition has biological roots. It is not imagined, it is not a lifestyle choice, and it is not going away on its own. Understanding the science behind it does not cure it, but it gives you something practical: the ability to ask better questions, to understand what the risks really are, to explain your needs clearly to the people who need to hear them, and to make informed decisions about your own care.
When you eat out, the most useful thing a restaurant can give you is specific, written, up-to-date allergen information — available before you order. When that information is there, you can apply everything you know about your own condition and make a real choice. That is what tools like Edible give the restaurants that use them. And that is what the biology says you need.
References
Renz H, Allen KJ, Sicherer SH, Sampson HA, Lack G, Beyer K, et al. Food allergy. Nat Rev Dis Primers. 2018;4:17098. DOI: 10.1038/nrdp.2017.98
Kelly MS, Bunyavanich S, Phipatanakul W, Lai PS. The environmental microbiome, allergic disease and asthma. J Allergy Clin Immunol Pract. 2022;10(9):2206–2217. DOI: 10.1016/j.jaip.2022.06.006
Du Toit G, Roberts G, Sayre PH, Bahnson HT, Radulovic S, Santos AF, et al. Randomized trial of peanut consumption in infants at risk for peanut allergy. N Engl J Med. 2015;372(9):803–813. DOI: 10.1056/NEJMoa1414850
Chun Y, Grishin A, Rose R, Zhao W, Arditi Z, Zhang L, Wood RA, Burks AW, Jones SM, Leung DYM, Jones DR, Sampson HA, Sicherer SH, Bunyavanich S. Longitudinal dynamics of the gut microbiome and metabolome in peanut allergy development. J Allergy Clin Immunol. 2023;152(6):1569–1580. DOI: 10.1016/j.jaci.2023.08.012
Lodge CJ, Lowe AJ, Allen KJ, et al. Efficacy and safety of oral immunotherapy for peanut, cow's milk, and hen's egg allergy: a systematic review of randomized controlled trials. Clin Transl Allergy. 2023;13(7):e12268. DOI: 10.1002/clt2.12268
