Allergies and intolerances: how the gut learns to tolerate food

Healthy state

Allergy and intolerance are two different things. In a true food allergy, the immune system reacts to a protein, usually via IgE antibodies IgE The antibody of classic allergy. It sits on mast cells and triggers their alarm on contact with the allergen. on mast cells Mast cells Immune cells beneath the skin and mucous membranes, filled with histamine and other messengers. They raise the alarm in allergies, infections and stress. More in the chapter “Mast cells: alarm cells in the gut, histamine and the disputed MCAS”. Even small amounts can trigger hives, swelling, breathing difficulties or circulatory shock. In an intolerance, usually an enzyme or a transporter is missing, for example with lactose, and the symptoms depend on the amount.

That we are not allergic to everything we eat is an achievement of the gut. Experts call it oral tolerance Oral tolerance The ability of the immune system to tolerate food protein taken up via the gut as harmless, mediated mainly by regulatory T cells. . Special immune cells in the gut lining take up food protein, carry it to the gut's lymph nodes and present it there to T cells, together with the signal “harmless”. This produces regulatory T cells, which dampen immune reactions against this protein throughout the body. In mice, this pathway needed retinoic acid from vitamin A and the messenger TGF-beta.

Gut bacteria help with this. Mice without gut flora Microbiome, gut microbiota The community of all microbes in the gut, mainly bacteria, plus viruses, fungi and archaea. Most live in the colon. More in the chapter “Building microbial diversity” or after antibiotics were more easily sensitised to food protein. One group of bacteria, the clostridia, was protective: they made the gut wall tighter and retuned certain immune cells. When researchers colonised germ-free mice with stool from healthy infants, the animals were protected against cow's milk allergy; with stool from infants with cow's milk allergy, they were not. A single bacterium, Anaerostipes caccae, a butyrate Butyrate A short-chain fatty acid and the main energy source for the cells of the colon wall. It strengthens the barrier and dampens inflammation. More in the chapter “Butyrate and other short-chain fatty acids” producer, was enough for part of the protection.

The route by which the immune system first meets a protein also matters. Via the gut, tolerance tends to develop; via inflamed skin, allergy tends to develop. This explains why children with atopic eczema more often develop food allergies, and why avoiding allergens does not protect.

Disrupted state: causes and effects

The best-known proof: in the LEAP study, 640 infants at high risk of allergy were given peanut either regularly or not at all. Among the children without initial sensitisation, at age five 13.7 per cent of the peanut avoiders had an allergy, but only 1.9 per cent of the peanut eaters. Eating early protected, avoiding did harm.

Acid blockers could disturb tolerance. Without stomach acid, proteins are broken down less and arrive in the gut in larger pieces. In 152 adults without known allergy, after three months of acid blockers, 15 per cent formed IgE antibodies against foods in a blood test for the first time; this did not confirm an allergy. In a cohort of almost 800,000 children, infants who were given acid blockers later had a food allergy about twice as often; after antibiotics in the first six months, asthma and hay fever were more common. These are associations; do not stop medically necessary acid blockers on your own because of them.

A gut infection can also trigger a local allergy in the gut. In mice, IgE antibodies against a protein eaten at the same time formed during an infection, only in the gut. Later, this protein caused abdominal pain via mast cells. In people with IBS Irritable bowel syndrome Recurring abdominal pain linked to bowel movements, without tests finding another cause. Today it is understood as a disorder of gut–brain interaction. More in the chapter “The gut-brain axis”, the gut lining swelled after injection of wheat, soy or milk protein. Such a local reaction cannot be seen in a blood test.

At the same time, far more people consider themselves allergic than actually are. In a European meta-analysis Meta-analysis An analysis that combines the results of several studies. It is only as good as the studies it includes. , 6 per cent reported a cow's milk allergy, but it was confirmed by a challenge test in 0.6 per cent. In IBS, 84 per cent of respondents reported symptoms from at least one food, mostly from poorly absorbed carbohydrates such as those in milk, beans or apples.

This matters because wrong diagnoses do harm. If you avoid many foods without reason, you eat a more one-sided diet, lose quality of life and feed your gut bacteria less well. IgG tests for foods only show what you have eaten and are not suitable for diagnosing allergies or intolerances.