Three Mechanisms, One Word Problem
Walk into any health food store and you will see "sensitivity tests" that claim to diagnose food reactions. Search online and you will find allergy, intolerance, and sensitivity used as if they mean the same thing. They do not. The distinction is immunological, and getting it wrong has real consequences — from missing a potentially fatal allergy to needlessly eliminating nutritious foods.
Food Allergy: The Immune System Overreacts
A true food allergy involves the adaptive immune system. In IgE-mediated allergy (type I hypersensitivity), first exposure sensitises the immune system to produce specific IgE antibodies against a food protein. On subsequent exposure, that IgE crosslinks on mast cell surfaces, triggering degranulation — histamine release, prostaglandins, leukotrienes — within minutes. Symptoms range from urticaria and angioedema to anaphylaxis. The top eight allergens (milk, egg, peanut, tree nuts, wheat, soy, fish, shellfish) account for about 90 percent of IgE-mediated food allergy, which affects roughly 2 to 4 percent of adults.
Non-IgE-mediated food allergy exists too. Food Protein-Induced Enterocolitis Syndrome (FPIES) triggers profuse vomiting and diarrhoea 2 to 6 hours after ingestion, mediated by T cells rather than IgE. It primarily affects infants and can cause hypovolaemic shock. Standard allergy skin prick tests and specific IgE blood tests are negative, making diagnosis challenging.
Food Intolerance: An Enzyme or Transport Problem
Food intolerance is non-immune. The most common example is lactose intolerance, where insufficient lactase enzyme leaves undigested lactose to be fermented by colonic bacteria. Other enzyme-based intolerances include sucrose intolerance (sucrase-isomaltase deficiency) and fructose malabsorption (limited GLUT5 transporter capacity). Pharmacological intolerances — caffeine sensitivity due to slow CYP1A2 metabolism, or histamine accumulation from diamine oxidase deficiency — also fall into this category.
Intolerances are typically dose-dependent: a splash of milk in tea may be fine, but a large glass triggers symptoms. Allergies, by contrast, can be triggered by trace amounts.
Food Sensitivity: The Grey Zone
"Food sensitivity" is the least well-defined category. Non-coeliac gluten sensitivity belongs here — patients report symptoms without identifiable immune or enzyme mechanisms, and nocebo effects are common in blinded challenges. Some researchers propose that innate immune activation (e.g., wheat amylase-trypsin inhibitors triggering TLR4) underpins certain sensitivities, but validated biomarkers do not yet exist.
The IgG Testing Problem
Commercial IgG food sensitivity panels are widely marketed but not validated for diagnosing food intolerance or sensitivity. IgG antibodies to foods are a normal physiological response to dietary exposure — they indicate you have eaten the food, not that you react to it. The European Academy of Allergy and Clinical Immunology, the American Academy of Allergy, and the Canadian Society of Allergy and Clinical Immunology have all issued position statements advising against IgG food testing for clinical decision-making.
The practical takeaway: suspected food allergy warrants specialist evaluation with validated tests (skin prick, specific IgE, oral food challenge). Suspected intolerance can be confirmed with breath tests or elimination-rechallenge. Sensitivity remains a clinical diagnosis requiring careful blinded challenge protocols.