Food allergy

A food allergy is a mistake made by the immune system: it reads the protein in an ordinary food as a threat and answers it the way it would answer an infection. Most of the time the whole thing stops at a rash, itching and an upset stomach, but in some people the same reaction unfolds within minutes and closes off the airway. That is why a food allergy cannot be filed alongside a delicate stomach: how bad the last episode was predicts the next one poorly.

What happens inside the body

At the first encounter the immune system memorises that food protein and makes antibodies against it; outwardly nothing happens. At later encounters those antibodies fire, cells release histamine, blood vessels widen, tissues swell and the muscle in the airways and the bowel tightens. That produces the whole range of symptoms, from weals on the skin to vomiting and a drop in blood pressure.

Why the immune system misfires on food in particular is still unknown. What is known is that the risk is higher if the person or a close relative has asthma, eczema or hay fever, and that children with severe eczema in the first year of life develop food allergy far more often.

There is also a second, slower mechanism that does not involve those antibodies. It causes not weals and swelling but persistent posseting, blood in the nappy, poor weight gain in a baby, or heavy vomiting several hours after a feed. These forms take longer to spot because the link with the food is not obvious.

What the reaction looks like

A reaction rarely stays in one place; usually several systems are involved at once.

• skin: itchy weals, redness, swelling of the lips, eyelids and face;
• mouth and throat: itching, tingling, a lump in the throat, a hoarse voice;
• breathing: coughing, wheezing, a blocked nose, sneezing, breathlessness;
• abdomen: nausea, vomiting, cramping pain, diarrhoea;
• general state: weakness, dizziness, a sense that something terrible is about to happen.

In classic allergy things start quickly: usually within the first few minutes and almost always within two hours of eating. The slower forms described above show themselves after hours or days, and there memory helps far less than written notes. If the trouble is limited to bloating, gurgling and stomach ache a few hours after a meal, with the skin and the breathing untouched, it points instead to food intolerance, which is not an immune reaction and does not cause anaphylaxis.

Signs of anaphylaxis and the first few minutes

Anaphylaxis is the reaction that affects breathing or circulation. It calls for action straight away.

• sudden swelling of the lips, tongue or throat, and a changed voice;
• difficult, noisy or very fast breathing, a feeling of choking;
• a tight throat or trouble swallowing;
• skin, lips or tongue turning blue, grey, pale or blotchy — on darker skin this shows up more readily on the palms and soles;
• sudden weakness, confusion, drowsiness, collapse;
• in a child, floppiness, limpness, no normal response to your voice.

If the person carries an adrenaline auto-injector, use it immediately, without waiting to see whether things settle on their own. The injection goes into the outer thigh, through clothing if need be. The instructions are printed on the device itself.

After the injection call an ambulance — in Spain, Italy, Portugal, Poland and Ukraine that is the single European number 112 — and say that this is anaphylaxis. Lay the person flat and raise their legs; if breathing is hard they will be more comfortable propped up, and if they are being sick, on their side. They must not stand or sit up abruptly even if they seem to be improving, because that alone can be dangerous. If there is no improvement after five minutes, a second dose is given from the spare device. Hospital assessment is needed either way: the reaction can return hours later without any fresh contact with the food.

When a familiar food suddenly turns on you

It happens that someone eats the same thing for years without trouble and then has a severe reaction. Usually there are circumstances that lower the threshold: exercise soon after eating, alcohol, painkillers from the non-steroidal anti-inflammatory group, an acute infection, too little sleep and, in women, certain days of the cycle.

Two rare but dangerous variants are worth knowing about, because almost nobody suspects them unaided.

• Exercise-dependent food allergy. The food — most often wheat, less often shellfish — is tolerated perfectly well, but if the person runs, trains or drinks alcohol in the hours after the meal, a severe reaction develops that can reach anaphylaxis. Until the link is found the attacks look like they come from nowhere.
• Delayed reaction to red meat. It follows tick bites and appears not at once but three to six hours after pork, beef or lamb, as night-time hives, swelling and abdominal pain. Because of that delay meat is the last thing anyone blames.

All of this is worth telling the doctor: without those details the investigation heads in the wrong direction.

The foods behind most reactions

Any food can cause an allergy, but a short list accounts for most cases: cow's milk, egg, peanut, tree nuts (walnut, hazelnut, almond, cashew, pistachio, Brazil nut), fish, crustaceans and molluscs, wheat, soya, sesame, mustard, celery and lupin flour in baked goods.

The childhood and adult lists differ. Milk and egg allergy resolves with time in most children, so it is reviewed periodically; allergy to peanut, tree nuts, fish and shellfish more often lasts for life. Within a group reactions are often cross-reactive: someone who reacts to prawn may react to crab, and someone who reacts to one nut may react to another. This is not something to test at home with a small bite.

Itchy mouth from a fresh apple

A separate and far more harmless story is the itching and slight tingling in the lips, mouth and throat brought on by raw fruit, vegetables or nuts, which fades within minutes. This is oral allergy syndrome: the immune system is not reacting to the fruit itself but to birch, mugwort or grass pollen, whose proteins resemble those in apple, cherry, peach, carrot and celery.

Those proteins are destroyed by heat, so a baked apple or cooked carrot goes down fine while the raw version does not. The symptoms almost always stay in the mouth. But if hives spread over the body, or swelling, abdominal pain or breathing difficulty joins the itching, this is no longer oral allergy syndrome and needs proper assessment.

How the diagnosis is confirmed

It all starts with a conversation: what was eaten, how long before symptoms began, what they looked like, how long they lasted, whether it has happened again. A food and symptom diary kept for a couple of weeks is worth more than any test ordered blind. The allergy specialist then chooses among several tools:

• skin prick tests: drops of extract are placed on the forearm and the skin is pricked lightly, with the result read after about a quarter of an hour;
• blood tests for antibodies to specific foods and sometimes to individual proteins within a food, which helps separate a serious allergy from cross-reaction with pollen;
• withdrawing the food for a few weeks and reintroducing it afterwards under medical supervision;
• a supervised food challenge: the food is given in increasing amounts in a unit equipped to treat a reaction. It is the most reliable method and the only one that can lift a diagnosis and put the food back on the plate.

Tests carry one important limit: a positive result on its own diagnoses nothing. It only shows that antibodies exist, whereas the illness exists when there are symptoms after eating. That is why hundred-food panels ordered without a clinical picture do harm — half a diet gets banned on the strength of them. Class G antibody tests, hair analysis, bioresonance and the drop of blood under a microscope do not detect food allergy at all.

Living with allergy without needless bans

The foundation is avoiding your own food and everything containing it. Across the European Union the commonest allergens must be named in the ingredient list and highlighted in the text, and catering premises must supply that information on request, so asking the waiter is both normal and your right.

What helps day to day:

• read the ingredients every time rather than once: manufacturers change recipes quietly;
• give notice in advance — to the cafe, the school and nursery, the workplace, whoever is hosting you, and the airline when booking;
• carry two adrenaline auto-injectors if they have been prescribed, and keep an eye on the expiry date;
• teach family, teachers and carers to use the device, because during a reaction the person may not be able to;
• keep a separate board and utensils at home and wash hands and surfaces before eating, since traces are sometimes enough.

Mild reactions are treated with antihistamines; severe ones with adrenaline alone, because an antihistamine neither stops anaphylaxis nor replaces the injection. Some children allergic to peanut and certain other foods are offered oral immunotherapy: tiny, gradually increasing amounts are given under supervision to raise the reaction threshold. It is not a cure, the food still has to be avoided, and the decision rests with the specialist.

The opposite rule matters just as much: do not cut foods out on the off chance, and do not put a child on restrictions without medical advice. Without milk, egg or wheat a diet quickly runs short of calcium, protein, iron and vitamins, and in children that shows up in their growth.

Online consultation

In an online consultation the doctor goes through the history of each reaction: what came before it, how fast it started, how it ended. That shows whether the picture fits allergy, oral allergy syndrome or intolerance, which tests are worth doing and in what order, and whether an adrenaline auto-injector is needed. If the picture is worrying, the doctor will say who to see in person and how urgently.

This material is for information only and does not replace medical advice.