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Food allergy, intolerance and IgG tests
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In one pass Feeling unwell after eating can come from two quite different sets of machinery in the body.
Educational content, not medical advice. See a clinician about your own case.
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Chapter 1
Two reactions, one test
Feeling unwell after eating can come from two quite different sets of machinery in the body. The 2023 guideline from the European Academy of Allergy and Clinical Immunology (EAACI) groups all adverse reactions to food under food hypersensitivity: when the immune system takes part it is food allergy, and when it does not, it is food intolerance.
Allergy is further split by the weapon the immune system uses: reactions driven by IgE antibodies, reactions that do not involve IgE, and a mixed type that has both. Intolerance happens at the level of digestion and metabolism instead: a missing enzyme, or substances in the food that act like a drug or occur in it naturally; some sugars also draw water into the gut and are fermented there by bacteria.
The IgG tests sold to the public measure something else: the immune system's normal memory of foods it has already met. No body of research supports using them to diagnose allergy or intolerance, and allergy societies advise against it.
If, after eating, the throat or tongue suddenly swells, breathing becomes hard, the skin or lips turn blue or grey, or the person faints, this is a severe allergic reaction: call emergency services immediately.
Allergy is further split by the weapon the immune system uses: reactions driven by IgE antibodies, reactions that do not involve IgE, and a mixed type that has both. Intolerance happens at the level of digestion and metabolism instead: a missing enzyme, or substances in the food that act like a drug or occur in it naturally; some sugars also draw water into the gut and are fermented there by bacteria.
The IgG tests sold to the public measure something else: the immune system's normal memory of foods it has already met. No body of research supports using them to diagnose allergy or intolerance, and allergy societies advise against it.
If, after eating, the throat or tongue suddenly swells, breathing becomes hard, the skin or lips turn blue or grey, or the person faints, this is a severe allergic reaction: call emergency services immediately.
Mechanism · Where the two kinds divide
The line is drawn at whether the immune system takes part. The 2010 guideline from the US National Institute of Allergy and Infectious Diseases (NIAID) defines food allergy as an adverse effect caused by a specific immune response to a food, one that recurs each time the food is eaten; a reaction with no established immune mechanism is an intolerance, and lactose intolerance is the classic example.The introduction to the UK NICE guideline for children lists where intolerance comes from: enzyme shortfalls, substances in food with a pharmacological action, and components that occur naturally in the food. The line matters because the two sides call for looking at different things: on one side the immune response, on the other digestion and how much was eaten. For background, a 2019 review quotes estimates of food allergy in about 1–2% of adults and under 10% of children, and intolerance in up to 20% of the population.
Chapter 2
IgE allergy: minutes to two hours
IgE allergy needs two things to be true at once. The first is sensitization: the blood carries IgE antibodies aimed specifically at a particular food, which a skin prick test or a blood test can pick up. The second is a typical reaction that comes on quickly after the food is eaten, such as sudden hives or swelling under the skin (angioedema).
The 2023 EAACI guideline puts it as two things at once for a diagnosis: typical symptoms that usually appear within 2 hours of exposure and recur on re-exposure, and evidence of IgE sensitization. The second on its own does not count: finding IgE shows sensitization, not allergy. The US NIAID guideline states plainly that sensitization alone is not enough to define food allergy.
The 2023 EAACI guideline puts it as two things at once for a diagnosis: typical symptoms that usually appear within 2 hours of exposure and recur on re-exposure, and evidence of IgE sensitization. The second on its own does not count: finding IgE shows sensitization, not allergy. The US NIAID guideline states plainly that sensitization alone is not enough to define food allergy.
Clinical · Exceptions to the two-hour rule
The 2023 EAACI guideline notes one exception to the two-hour rule: in people allergic to alpha-gal in red meat, symptoms often start 3 to 6 hours after eating.In other cases a reaction needs a co-factor before it shows up. The co-factors the guideline lists include exercise, alcohol, non-steroidal anti-inflammatory drugs () such as aspirin, acid-suppressing medicines and lack of sleep, and each should be asked about when the history is taken.
Clinical · Telling sensitization from allergy
A skin prick test and a blood test for food-specific IgE (sIgE) both measure sensitization. The 2010 NIAID guideline holds that neither one, on its own, can diagnose food allergy. The 2023 EAACI guideline makes them the first-line tests (a strong recommendation), but only after the history has been taken and points to a particular food.Sensitization and allergy really do come apart in real bodies: in a 2015 peanut study, 77 children had peanut IgE in their blood yet ate peanut as usual with no reaction.
The deciding test is the oral food challenge: the suspect food is eaten under medical supervision while staff watch for a reaction. NIAID calls the double-blind, placebo-controlled challenge the gold standard; EAACI 2023 makes the supervised oral challenge the reference standard and considers an open challenge adequate in most cases.
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Red flag · Anaphylaxis
The most dangerous IgE reaction is anaphylaxis. Treat these signs as an emergency: the throat or tongue suddenly swells, breathing becomes difficult or very fast, the skin, lips or tongue turn blue or grey, or the person feels faint or collapses. The NHS advice is to use an adrenaline auto-injector straight away if there is one, and call emergency services immediately.The 2020 World Allergy Organization (WAO) guidance makes intramuscular adrenaline (epinephrine) the first-line treatment; antihistamines only ease skin symptoms and cannot replace it. It also helps to know that 10–20% of anaphylaxis episodes come without a rash, so clear skin does not mean all is well.
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Chapter 3
The slow ones: non-IgE allergy and intolerance
Not every food reaction is fast. Non-IgE allergy is still an immune reaction, but it does not run through IgE; many of these reactions are thought to be driven directly by T cells. So symptoms usually come later and gather in the gut and the skin: reflux, loose or frequent stools, blood or mucus in the stool, tummy pain, eczema, and in babies refusing feeds or growing slowly.
Intolerance involves no immune system at all, and how bad the symptoms are usually depends on how much was eaten. There are three common causes: not enough of the enzyme that digests a particular sugar; certain sugars pulling water into the gut and then being fermented by bacteria into gas; or something in the food itself that acts like a drug, such as caffeine.
Intolerance involves no immune system at all, and how bad the symptoms are usually depends on how much was eaten. There are three common causes: not enough of the enzyme that digests a particular sugar; certain sugars pulling water into the gut and then being fermented by bacteria into gas; or something in the food itself that acts like a drug, such as caffeine.
Clinical · Gut allergies in babies
The 2010 NIAID guideline lists several gut allergies that do not involve IgE: food protein-induced enterocolitis syndrome (FPIES), allergic proctocolitis, and eosinophilic esophagitis; eosinophilic esophagitis can only be confirmed by taking a biopsy during endoscopy and examining the tissue.FPIES is seen mainly in babies. In an acute episode there is repeated vomiting, typically 1–4 hours after the trigger food, and some children go on to have diarrhea within 5–10 hours. It lacks the usual skin and breathing signs of allergy, and most children have a negative skin prick test and no detectable specific IgE. Common triggers include cow's milk, grains (rice, oats), soy and fish, varying by country; diagnosis rests on the clinical picture, with an oral food challenge when it is unclear.
Many children grow out of it, usually earlier for milk and soy than for grains, but the timing varies widely by food and by country and has not been studied systematically.
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Mechanism · Lactose and FODMAPs
Lactose intolerance is the classic enzyme shortfall: there is not enough lactase, the enzyme in the small intestine that breaks lactose down, so undigested lactose travels on to the colon, where bacteria ferment it.are a group of short-chain carbohydrates that the small intestine absorbs poorly. They pull water into the gut (an osmotic effect) and are fermented by bacteria into gas, and both stretch the bowel; in people whose gut is already more sensitive to stretch (visceral hypersensitivity), that stretch is felt as bloating and pain. The review rates this mechanism as the best supported of all the intolerances.
Both reactions depend on dose, so the usual approach is to cut back first and then add the food back step by step, to find out how much is tolerated. Restrictions such as a low-FODMAP diet are best done with a dietitian, who checks that the diet still provides enough and that the list of cut foods does not keep growing.
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Evidence · Histamine and gluten
Some intolerances are talked about far more than the evidence supports. Histamine intolerance has no validated biomarker so far: blood tests for diamine oxidase (DAO) or histamine are scientifically inconclusive, and the review advises against using them routinely. The international urticaria guideline likewise warns against overdiagnosing histamine intolerance; in chronic hives it reserves a low-histamine diet for a short trial in a minority of patients.The same goes for non-coeliac gluten sensitivity: when two groups of people who believed they reacted to gluten were given double-blind challenges, only 5% and 14% actually reacted to gluten. The discomfort they feel is real, but it may not come from gluten itself.
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Chapter 4
What an IgG test actually measures
The immune system meets the food we eat every day. For nearly all foods, the body's response is to recognize it and then tolerate it, and along the way it makes IgG antibodies against that food, including the IgG4 subclass. So food IgG is closer to a mark that says met before, tolerated: healthy adults and children are expected to test positive, and foods eaten again and again tend to give higher values.
In tolerant people, IgG4 may even work as a brake, as laboratory work from a peanut study suggests. Reading IgG values as a diagnosis of allergy or intolerance turns a mark of tolerance into a mark of disease, which is why several allergy societies explicitly advise against these tests.
In tolerant people, IgG4 may even work as a brake, as laboratory work from a peanut study suggests. Reading IgG values as a diagnosis of allergy or intolerance turns a mark of tolerance into a mark of disease, which is why several allergy societies explicitly advise against these tests.
Evidence · IgG4 in a peanut study
In the 2015 peanut study mentioned earlier (228 children in all), the sensitized-but-tolerant children had higher peanut IgG4 and a higher ratio of IgG4 to IgE.In the laboratory, plasma from tolerant children held back peanut-induced activation of basophils and mast cells; when IgG4 was removed from the plasma, the median inhibition fell from 75% to 30%. In children given peanut oral immunotherapy, IgE did not change while peanut IgG4 rose about 12-fold.
The limits need stating: these are laboratory experiments, in children, on peanut only. The authors say outright that their design cannot answer whether IgG4 is useful for diagnosis; the limitations they name include that avoiding the allergen may affect IgG4 levels, and that IgG1 was not measured.
Evidence · One trial that came out positive
A 2004 in the journal Gut tested avoidance guided by IgG results in 150 outpatients with irritable bowel syndrome (). For 12 weeks, one group cut out the foods their IgG results flagged, and the other cut out the same number of foods that did not match the results (a sham diet).The result: symptom scores fell 10% more on the true diet than on the sham diet (mean difference 39, 95% 5–72); among those who kept fully to the diet, the gap was 26%.
This positive result has to be read together with its conditions. The test covered 29 foods and was run by a commercial laboratory (YorkTest), which also wrote the diet sheets; more people dropped out of the true diet (24 against 13); the fully adherent subgroup with the largest gap was not formed by randomization; and the sham group improved too. The authors concluded that the approach deserves further research, not that the test has been validated.
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Safety · Why the guidelines say no
Opinion against these tests is close to unanimous. A 2008 EAACI task force report concluded that food IgG4 testing should not be used for diagnosis. The 2023 EAACI guideline strongly recommends against isolated IgG and IgG-subclass tests; the certainty of evidence is very low, and the recommendation rests on the tests lacking any rationale while carrying clear risks. The UK NICE guideline for children says plainly not to use serum-specific IgG to diagnose food allergy. The 2010 US NIAID guideline lists allergen-specific IgG4 among the nonstandardized tests that should not be used. The American Academy of Allergy, Asthma & Immunology (AAAAI) and the Canadian Society of Allergy and Clinical Immunology (CSACI) both advise against using it to look for allergy or intolerance.The risk is avoidance that is not needed. Cutting out foods because a test report says so can leave children growing poorly or malnourished; the US NIAID guideline likewise warns that when people without a proven allergy avoid foods, they risk nutritional deficiencies and poor growth. Separately, prolonged diets that remove several foods at once have been reported, in case reports, to cause severe malnutrition. The opposite direction is dangerous too: someone with a true IgE allergy may not have raised IgG and may be wrongly told the food is safe to eat again.
Chapter 5
Three questions that point to the right clinic
Pull the mechanisms together and you get the things a doctor most wants to know when taking a history:
1. How fast does it come on? Within minutes to two hours looks more like IgE allergy; several hours later points more to a non-IgE reaction.
2. What are the symptoms? Hives, sudden swelling of the skin or wheezing point to IgE allergy; bloating, diarrhea or reflux centred on the gut are more common in the other two kinds.
3. Does the amount matter? Reactions that depend on the amount are more common with intolerance, but an IgE allergy can also flare only after a larger amount, or when exercise or alcohol is added; whenever hives, swelling or wheezing appear, treat it as allergy, however little was eaten.
Add one more: does it happen every time? If the throat or tongue swells or breathing becomes hard, do not wait for a clinic: call emergency services immediately. Beyond that, here is this article's own rough guide, not a rule from the guidelines: reactions that come fast and involve the skin can go first to an allergy clinic; reactions that come slowly, depend on how much is eaten and stay in the gut can go first to a digestive (gastroenterology) clinic. This only helps you decide where to go first and how to describe what happens; it is not a self-diagnosis, and the guidelines also oppose cutting out large parts of the diet on the strength of a test report alone.
1. How fast does it come on? Within minutes to two hours looks more like IgE allergy; several hours later points more to a non-IgE reaction.
2. What are the symptoms? Hives, sudden swelling of the skin or wheezing point to IgE allergy; bloating, diarrhea or reflux centred on the gut are more common in the other two kinds.
3. Does the amount matter? Reactions that depend on the amount are more common with intolerance, but an IgE allergy can also flare only after a larger amount, or when exercise or alcohol is added; whenever hives, swelling or wheezing appear, treat it as allergy, however little was eaten.
Add one more: does it happen every time? If the throat or tongue swells or breathing becomes hard, do not wait for a clinic: call emergency services immediately. Beyond that, here is this article's own rough guide, not a rule from the guidelines: reactions that come fast and involve the skin can go first to an allergy clinic; reactions that come slowly, depend on how much is eaten and stay in the gut can go first to a digestive (gastroenterology) clinic. This only helps you decide where to go first and how to describe what happens; it is not a self-diagnosis, and the guidelines also oppose cutting out large parts of the diet on the strength of a test report alone.
In practice · What to note before the visit
The 2023 EAACI history checklist (its Table 5) works as a template for notes:what the symptoms were, and where in the body;how long after eating they began, and how long they lasted;how much was eaten: one bite or a full serving;whether it happens every time;whether the food was raw, cooked or processed;whether there was exercise, alcohol, or a painkiller or similar medicine around that time.
The UK NICE checklist is almost the same, and adds how severe it was and how often it happens. NICE is also clear that allergy tests are not done until that history has been taken.
Clinical · Stop it, then bring it back
When non-IgE allergy is suspected, NICE uses elimination and reintroduction: stop the suspect food, normally for 2–6 weeks, then bring it back and see whether the symptoms go away and then return; a dietitian should check throughout that the diet still provides enough nutrition. This guideline covers children and young people under 19.NICE lists the situations that call for a specialist referral, including a past acute systemic reaction, a severe delayed reaction, faltering growth, or suspected allergy to several foods.
A diagnosis is not permanent either: the 2023 EAACI guideline suggests reassessing children with confirmed food allergy from time to time, because some of them outgrow it. This is only a conditional recommendation, there are no controlled studies on how often to reassess, and resolution in adults is considered unlikely.
In practice · One table to compare them
Putting the criteria side by side:IgE allergy: minutes to about two hours; hives, swelling, wheezing, vomiting; returns each time the food is eaten; history first, then a skin prick test or sIgE, with an oral food challenge to confirm when needed.Non-IgE allergy: delayed; mostly in the gut and skin; in FPIES, for example, skin prick tests and sIgE are usually negative; worked out with a stop-then-reintroduce diet trial.Intolerance: no immune system involved; depends on how much is eaten; no reliable test, worked out by cutting back and trying again.IgG tests: reflect exposure and tolerance, and can diagnose none of the three above.
This table only helps you decide where to go first and how to describe things to a doctor; it is not a self-diagnosis.
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References · 9
- Santos, A. F., Riggioni, C., Agache, I., Akdis, C. A., Akdis, M., Alvarez-Perea, A., et al. (2023). EAACI guidelines on the diagnosis of IgE-mediated food allergy. Allergy, 78(12), 3057-3076. 10.1111/all.15902
- National Institute for Health and Care Excellence. (2011, updated 2018). Food allergy in under 19s: assessment and diagnosis (Clinical guideline CG116). www.nice.org.uk/guidance/cg116
- Tuck, C. J., Biesiekierski, J. R., Schmid-Grendelmeier, P., & Pohl, D. (2019). Food intolerances. Nutrients, 11(7), 1684. 10.3390/nu11071684
- Carr, S., Chan, E., Lavine, E., & Moote, W. (2012). CSACI position statement on the testing of food-specific IgG. Allergy, Asthma & Clinical Immunology, 8(1), 12. 10.1186/1710-1492-8-12
- American Academy of Allergy, Asthma & Immunology. (n.d.). The myth of IgG food panel testing. www.aaaai.org/tools-for-the-public/conditions-library/allergies/igg-food-test
- NHS. (2023). Anaphylaxis (page last reviewed 21 June 2023). www.nhs.uk/conditions/anaphylaxis
- Boyce, J. A., Assa'ad, A., Burks, A. W., Jones, S. M., Sampson, H. A., Wood, R. A., et al.; NIAID-Sponsored Expert Panel. (2010). Guidelines for the diagnosis and management of food allergy in the United States: Report of the NIAID-sponsored expert panel. Journal of Allergy and Clinical Immunology, 126(6 Suppl), S1-S58. 10.1016/j.jaci.2010.10.007
- Stapel, S. O., Asero, R., Ballmer-Weber, B. K., Knol, E. F., Strobel, S., Vieths, S., & Kleine-Tebbe, J.; EAACI Task Force. (2008). Testing for IgG4 against foods is not recommended as a diagnostic tool: EAACI Task Force Report. Allergy, 63(7), 793-796. 10.1111/j.1398-9995.2008.01705.x
- Santos, A. F., James, L. K., Bahnson, H. T., Shamji, M. H., Couto-Francisco, N. C., Islam, S., Houghton, S., Clark, A. T., Stephens, A., Turcanu, V., Durham, S. R., Gould, H. J., & Lack, G. (2015). IgG4 inhibits peanut-induced basophil and mast cell activation in peanut-tolerant children sensitized to peanut major allergens. Journal of Allergy and Clinical Immunology, 135(5), 1249-1256. 10.1016/j.jaci.2015.01.012
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