Food allergy, intolerance and IgG tests

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Two reactions, one test

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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.

Educational content, not medical advice. See a clinician about your own case.

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