A food allergy is an immune reaction. It usually begins within minutes of eating, a trace of the food can set it off, and at its most severe it causes anaphylaxis, which can be fatal without emergency treatment. A food intolerance happens when the gut struggles to break a food down. Symptoms build over hours rather than minutes, they scale with how much you ate, and they are unpleasant rather than dangerous. This guide covers how each reaction works, the emergency signs to recognise, how allergy is tested in the UK, and what blood tests for food reactions can and cannot tell you.
Call 999 if any of these develop after eating.
- Swelling of the lips, mouth, tongue or throat
- Breathing very fast, struggling to breathe, or wheezing
- A tight throat, or struggling to swallow
- Skin, lips or tongue turning blue, grey or pale
- Sudden confusion, drowsiness, dizziness or fainting
- In a child, going limp, floppy or unresponsive
A raised, itchy rash often appears alongside these. They are signs of anaphylaxis, and anaphylaxis needs an ambulance, not a GP appointment and not a test of any kind. If an adrenaline pen such as an EpiPen has been prescribed, use it straight away, then call 999 even if the person seems to recover, because symptoms can return.
What happens in a food allergy?
In the most common form of food allergy, the immune system mistakes a food protein for a threat and makes an antibody called IgE against it. The next time the food arrives, that IgE triggers an immediate release of histamine and other chemicals. This is why these reactions are fast, usually starting within minutes and rarely more than two hours after eating, and why a tiny amount can be enough. Reactions range from an itchy mouth, hives and swelling through to vomiting, breathing difficulty and anaphylaxis.
A smaller group of allergies does not involve IgE. These non-IgE reactions develop over hours or even days, mostly in babies and young children reacting to cows' milk, and they are one reason a child's reactions to food always belong with a GP rather than home detective work.
Confirmed food allergy is less common than most people believe. A 2023 pooled analysis of European studies found that around one in five people report a food allergy, while under one in a hundred had one confirmed when researchers tested them with supervised feeding tests. The gap is not imagined symptoms. It is mostly intolerances and other gut conditions being read as allergy, which is why working out which one you have matters.
What happens in a food intolerance?
An intolerance does not involve IgE and does not touch the airways or circulation. In most cases the gut lacks enough of an enzyme to digest part of a food. The undigested remainder reaches the colon, where gut bacteria ferment it, and the gas and fluid this generates produce bloating, cramps, wind, loose stools and often tiredness, arriving anywhere from half an hour to many hours after eating. What a food intolerance is depends on the mechanism behind it, and food intolerance symptoms cover a wider range than most people expect.
Lactose intolerance is the clearest example. In most of the world's adults, production of lactase, the enzyme that digests milk sugar, falls away after childhood. Diagnosis rests on a hydrogen breath test alongside your symptom history, which a 2025 review describes as the standard approach. No blood test can diagnose lactose intolerance, including ours.
How do you tell the difference?
Speed, dose and the type of symptom carry most of the answer.
| Food allergy | Food intolerance | |
|---|---|---|
| How fast it starts | Usually within minutes of the food | A slow build over hours, often through the afternoon or evening |
| How much it takes | A crumb, a splash of milk or a kiss from someone who has eaten the food can be enough | A little is often fine and a lot is not |
| Where it shows | Hives, swelling, an itchy mouth and any change to breathing | Mostly the gut, sometimes with headaches or tiredness |
The overlap sits in the gut, because both can cause pain, nausea and diarrhoea. The pattern around a reaction therefore tells you more than any single symptom. If a reaction has ever involved breathing changes, swelling or widespread hives, treat it as a possible allergy and take it to your GP. Do not investigate that question at home with any kit, ours included.
Which allergens does UK labelling law cover?
UK food law recognises 14 allergens behind most serious reactions. They are celery, cereals containing gluten, crustaceans such as prawns and crab, eggs, fish, lupin, milk, molluscs such as mussels, mustard, peanuts, sesame, soybeans, sulphur dioxide and sulphites, and tree nuts. Prepacked food must emphasise them in the ingredients list, usually in bold, and since 2021 food packed on the premises for direct sale, such as a bakery sandwich, must carry a full ingredients label too. The Food Standards Agency's advice for consumers covers the list and what to look for when shopping or eating out.
Some of the 14 sit on both sides of the line. Milk can be an allergy, most often in young children, or a lactose intolerance, most often in adults, and the two mechanisms are entirely different, a reaction to milk protein against a missing enzyme for milk sugar. Sulphites mainly trigger asthma-type reactions. Peanuts, tree nuts, sesame, fish and shellfish are different again. These foods cause most severe reactions and have no common intolerance mechanism, so a reaction after eating them is an allergy question first, whatever word you would instinctively use for it.
How is a food allergy tested in the UK?
Start with your GP. NICE guidance on food allergy in children and young people asks GPs to take a detailed, allergy-focused history first, then test only for the foods that history points to, and the same logic is applied to adults. The NHS uses two main tests. A skin prick test places a drop of the food extract on the forearm to see whether the skin reacts, and a blood test measures IgE against specific foods. Where reactions have been severe or the picture is unclear, the GP refers to an allergy clinic.
Neither test settles the question alone, because you can carry IgE against a food you eat without any trouble. Where doubt remains, the reference test in the 2023 European allergy guideline is an oral food challenge, eating gradually increasing amounts of the food under medical supervision. The supervision is what makes it safe, and it is never something to attempt at home.
A GP will not order an IgG food panel, hair analysis or kinesiology to investigate allergy, because none of them can detect it.
Where does an IgG test like ours fit?
Our IgG food intolerance test measures IgG antibodies against more than 200 foods and drinks from a small blood sample. IgG is a different antibody from the IgE behind allergic reactions, and one does not stand in for the other. The test does not analyse IgE allergies, it cannot diagnose lactose intolerance or coeliac disease, and no result from it can ever tell you a food is safe after an allergic reaction.
Allergy organisations have examined IgG testing and reached the same conclusion in Europe and North America. A European task force report and a Canadian position statement both state that IgG results should not be used to diagnose food allergy. We agree, and our own test report states the same limits.
What an elevated IgG result may indicate is a food sensitivity, a food-specific antibody reaction that is distinct from both an IgE allergy and a digestive intolerance. Used that way, the panel gives you a shortlist of foods worth examining rather than a verdict, and our clinical team interprets results in exactly that spirit. Any removing and reintroducing of foods that follows should be structured and supported, by our team or a registered dietitian, because removing whole food groups for months on end without guidance risks nutritional shortfalls. A 2023 review found that most reported food reactions are self-diagnosed rather than confirmed, and that unnecessary restriction is the usual result. A structured process exists to prevent both.
Do not cut out gluten before a coeliac test
Coeliac disease is a third category. It is an autoimmune condition in which gluten triggers the immune system to damage the small intestine, so it is neither an allergy in the IgE sense nor an intolerance, and it has its own NHS diagnostic pathway of a blood test followed by a biopsy where needed. The blood test can only detect coeliac disease while gluten is still part of your regular diet. If bread and pasta seem to be behind your symptoms, see your GP for coeliac testing before you change anything about your diet, because going gluten-free first can make the test read falsely negative and leave you without a diagnosis that matters for life, including for close family, since the condition runs in families.
If coeliac disease has been ruled out and wheat still seems to be a trigger, gluten intolerance, sometimes called non-coeliac gluten sensitivity, is the next possibility to look into, and the evidence behind it is less settled than many pages suggest.
When should you see a GP?
Book a GP appointment, rather than reaching for any home test, if any of the following applies.
- Any reaction to food that involved swelling, hives, an itchy mouth or a change to breathing, however mild it seemed at the time
- Any suspected reaction in a child. Children's reactions are assessed by a GP or a paediatric allergy service, and foods are never removed from a child's diet without professional advice
- Symptoms that could point to coeliac disease, before any change to your diet
- Blood in your stools, unexplained weight loss, a change in bowel habit lasting more than three weeks, difficulty swallowing, persistent vomiting, or new persistent gut symptoms over the age of 50. These need investigating properly before any food explanation is considered
And for the anaphylaxis signs listed above, swelling of the lips, tongue or throat, difficulty breathing, or feeling faint after eating, call 999 at the time it happens, not afterwards.
This guide is for information only and does not replace medical advice. Speak to your GP or pharmacist about your own symptoms.
Sources
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- Santos AF, Riggioni C, Agache I, et al. (2023). EAACI guidelines on the diagnosis of IgE-mediated food allergy. Allergy. PubMed | DOI
- Stapel SO, Asero R, Ballmer-Weber BK, et al. (2008). Testing for IgG4 against foods is not recommended as a diagnostic tool: EAACI Task Force Report. Allergy. PubMed | DOI
- Carr S, Chan E, Lavine E, Moote W (2012). CSACI Position statement on the testing of food-specific IgG. Allergy, Asthma & Clinical Immunology. PubMed | DOI
- Zingone F, Bertin L, Maniero D, et al. (2023). Myths and Facts about Food Intolerance: A Narrative Review. Nutrients. PubMed | DOI
- Borralho AI, Marcos P (2025). Lactose Intolerance and Malabsorption Revisited: Exploring the Impact and Solutions. GE Portuguese Journal of Gastroenterology. PubMed | DOI
- NICE (2011). Food allergy in under 19s: assessment and diagnosis. Clinical guideline CG116. nice.org.uk
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- NHS. Anaphylaxis. nhs.uk
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- Food Standards Agency. Food allergy and intolerance advice for consumers. gov.uk
