If you have ever had a "food sensitivity" test — the kind sold online or in health food shops that measures your IgG levels against dozens of foods — you may have been told you are sensitive to things you eat every day. Milk. Wheat. Eggs. The results can look alarming. But what do they actually mean?
In Part 1, we covered how IgE-mediated food allergy works: your immune system producing antibodies against a food protein, those antibodies arming your mast cells, and on re-exposure, a chemical cascade that can become life-threatening within minutes. This post is about IgG — a different antibody that comes up a lot in conversations about food sensitivity — what it actually tells you, and why the distinction between IgE and IgG matters for how you understand your own condition.
Two antibodies, completely different jobs
Antibodies are Y-shaped proteins made by your immune system. They recognise specific targets on harmful cells or substances and help coordinate the immune response. There are several types, each with a different role. IgE and IgG are two of them, but they do almost opposite things when it comes to food.
IgE, as we saw in Part 1, is the antibody at the centre of fast, potentially life-threatening allergic reactions. When IgE on a mast cell binds its target allergen, the cell releases its chemicals within seconds to minutes.
IgG works differently. When your immune system is exposed to a food protein over and over again, it makes IgG antibodies against it. But this does not mean you are allergic or intolerant to that food. Food-specific IgG4 reflects a normal response after repeated exposure to food, linked to regulatory immune cells that promote tolerance. Testing for IgG4 against foods is considered irrelevant to diagnosing food allergy or intolerance [1].
Put simply: a positive IgG result against a food mostly tells you which foods you have been eating regularly. It is a sign of exposure, and often of tolerance — your immune system learning to accept that food, not gearing up to attack it.
The problem with "food sensitivity" tests
A large industry has grown up around blood tests that measure IgG levels against dozens or hundreds of foods and present positive results as evidence of "food sensitivities" requiring dietary changes. If you have had one of these tests, you may have been told to cut out foods you had no problem eating.
Every major clinical allergy body has taken a clear position on this. The European Academy of Allergy and Clinical Immunology (EAACI) states that IgG4 antibody levels indicate exposure, not disease [1]. The American Academy of Allergy, Asthma and Immunology (AAAAI) supports that position [2]. The Canadian Society of Allergy and Clinical Immunology (CSACI) concluded that positive IgG results for foods are to be expected in normal, healthy people, and that using these tests inappropriately leads to false diagnoses, unnecessary dietary restrictions, and poorer quality of life [3].
None of this means your symptoms are not real. It means the IgG test is not a reliable map of what is causing them. If you are experiencing symptoms you think are food-related, a conversation with your GP or a referral to a clinical allergy service will get you further than a test bought online.
So what is a food intolerance?
Food allergy is immune-mediated. Food intolerance is not. Most food intolerances happen because your body lacks the right enzyme to process a particular ingredient, and the effects are felt in your gut rather than through an immune cascade.
Lactose intolerance is the clearest example. Lactose is the sugar in milk and dairy products. To digest it, your body needs an enzyme called lactase, which breaks lactose down into two simpler sugars that can then be absorbed. When your gut does not make enough lactase, undigested lactose passes into your large intestine, where bacteria ferment it, producing gas and drawing in fluid. The result is bloating, cramping, and diarrhoea [4]. No antibodies. No mast cells. No immune response. The problem is a missing or insufficient enzyme.
About 65% of the world's adult population has reduced lactase activity after childhood [4]. The ability to keep digesting milk into adulthood, which is common in people of northern European ancestry, is the genetic exception rather than the norm.
What this means for you
The IgE/enzyme distinction matters because it changes what you need to worry about and what you can afford to be flexible with.
If you have a milk allergy (IgE-mediated), your immune system reacts to proteins in milk — usually casein or whey. This is a true allergy, with the potential for anaphylaxis. You need to know whether milk protein is present anywhere in a dish, including in stocks, sauces, or anything cooked in butter.
If you have lactose intolerance (enzyme deficiency), you may find you can tolerate hard aged cheeses, which are very low in lactose because most is lost during maturation, or small amounts of dairy in a cooked dish. You have probably been managing this for years and have a good sense of your own threshold. The risk is discomfort, not anaphylaxis.
Knowing which of these applies to you changes the questions you ask at a restaurant, the level of detail you need from a menu, and the degree of risk you are actually managing. For a confirmed IgE allergy, you need full allergen protocol: specific ingredients, cross-contact information, confident answers. For an intolerance, detailed ingredient information helps you make a choice based on what you know about your own body.
Both situations are served better by a menu that says clearly what is in every dish than by a conversation that depends on whoever happens to be working that shift.
The same logic applies across your diet
The allergy/intolerance distinction runs through most of the dietary conditions you might be managing. Coeliac disease is an autoimmune condition the lining of your small intestine in response to gluten — it requires strict, permanent gluten avoidance and careful attention to cross-contact. Non-coeliac gluten sensitivity produces gluten-related symptoms without that immune response. Wheat allergy is an IgE-mediated reaction to wheat proteins specifically, with the potential for anaphylaxis.
If you tell a restaurant "I can't have gluten", you could be describing any of these. The right response depends on which one applies. Knowing your own diagnosis, and being able to say clearly what you need — "I have coeliac disease, so I need to avoid all gluten including cross-contact" or "I have a wheat allergy" — helps the restaurant give you the right answer. Understanding the biology behind your condition is not just academic. It makes you a better communicator of your own needs.
Coming in Part 3: cross-reactivity — why your allergy might extend further than you think, and how shared protein structures between different foods create risks that a single allergen declaration can miss.
References
Stapel SO, Asero R, Ballmer-Weber BK, Knol EF, Strobel S, Vieths S, et al. Testing for IgG4 against foods is not recommended as a diagnostic tool: EAACI Task Force Report. Allergy. 2008;63(7):793–796. DOI: 10.1111/j.1398-9995.2008.01705.x
Bock SA. AAAAI support of the EAACI Position Paper on IgG4. J Allergy Clin Immunol. 2010;125(6):1410. DOI: 10.1016/j.jaci.2010.03.013
Carr S, Chan E, Lavine E, Moote W. CSACI position statement on the testing of food-specific IgG. Allergy Asthma Clin Immunol. 2012;8(1):12. DOI: 10.1186/1710-1492-8-12
Szilagyi A, Ishayek N. Lactose intolerance, dairy avoidance, and treatment options. Nutrients. 2018;10(12):1994. DOI: 10.3390/nu10121994
