What is an allergy?

The definition, without the circle

Allergy is usually defined as a hypersensitive reaction to an allergen, and an allergen as a substance that causes an allergic reaction. That is circular and explains nothing, so it is worth unpicking properly.

An allergy is a specific malfunction: the immune system identifies a harmless substance as a threat and mounts a defence against it. The substance is not dangerous and the defence is not needed. The symptoms are not caused by the substance at all — they are caused by the body's own response to it.

That last point explains a great deal. It is why a single peanut can do more harm than a large quantity of something genuinely toxic, and why two people meeting the same pollen on the same afternoon can have entirely different days.

What the immune system is doing

The immune system's ordinary job is to distinguish the body's own cells from everything else. Every cell carries molecular markers identifying it as belonging to this body; bacteria, viruses and transplanted tissue do not match, and are treated as foreign and removed.

Much of the machinery for this sits in the lymphatic system, which runs in parallel with the blood vessels: the lymph nodes, the bone marrow, the spleen and the thymus, along with lymphoid tissue in the tonsils and the gut. White blood cells of various kinds patrol the blood and lymph, identifying what is foreign, flagging it for removal, and in some cases attacking it directly.

In the most common form of allergy, the sequence runs like this. On an early encounter with the substance, the immune system produces an antibody called IgE — immunoglobulin E — directed specifically at it. That IgE attaches to mast cells, which sit resident in the tissues of the skin, the airway lining and the gut, and to basophils circulating in the blood. The person now has IgE primed against that substance and ready. This stage is called sensitisation, and on its own it produces no symptoms whatever.

On a later encounter, the substance binds to the IgE already in place, and the mast cells release their contents into the surrounding tissue — histamine chiefly, along with prostaglandins and other compounds. Those cause blood vessels to leak and smooth muscle to contract, which is what produces the swelling, redness, itching, streaming and wheezing. This is why reactions of this type are fast: the antibody is already waiting, so nothing has to be built from scratch.

Not all allergy works this way. Some reactions, including much contact dermatitis and a proportion of food allergy in infants, run on slower mechanisms that do not involve IgE at all, take hours or days to appear, and do not show up on the usual allergy tests.

What people react to

Allergens are usually proteins, and the common ones are a short list: pollen from grasses, trees and weeds; the waste particles of house dust mites; proteins shed in the skin flakes, saliva and urine of animals; mould spores; insect venom; latex; and a handful of foods, with cow's milk, egg, peanut, tree nuts, fish, shellfish, wheat, soya and sesame accounting for most food allergy.

Some substances cause trouble by a different route — acting directly on the skin, or irritating the airway — without any immune involvement. Those are not allergies, however similar the discomfort.

Why some people and not others

The tendency to develop allergies has a name, atopy, and it runs in families. Someone with two atopic parents is substantially more likely to be atopic themselves, and people who are atopic often collect several related conditions over a lifetime: eczema in infancy, then food allergy, then asthma and hay fever.

No single gene accounts for it, and heredity is not the whole story. The sharp rise in allergy across industrialised countries over a few generations is far too fast to be genetic, which means something in the environment is doing a great deal of the work. Which something remains an open question, and the honest position is that it is not yet settled.

How an allergy develops

Allergies are not necessarily lifelong, and they are not necessarily present from birth. They can appear in adulthood to something tolerated for decades, and some — particularly cow's milk and egg allergy in children — are outgrown.

House dust mite allergy illustrates how one builds. The allergen is not the mite itself but proteins in the particles of waste it leaves in mattresses, bedding, carpets and soft furnishings. Disturbing those surfaces lifts the particles into the air to be breathed in. Many people inhale them for a lifetime without consequence. In others the immune system gradually produces IgE against those proteins, and at some point the exposure that was once unremarkable starts to produce a reaction instead.

What a reaction looks like

The same underlying process presents very differently depending on where it happens.

In the nose and sinuses it is allergic rhinitis — sneezing, congestion, a streaming nose — and seasonally, hay fever. In the eyes it is allergic conjunctivitis, with redness, watering and itching. In the skin it appears as hives, or as the itching and inflammation of eczema. In the airway it can trigger asthma. In the gut it causes pain, vomiting and diarrhoea.

Most of this is uncomfortable rather than dangerous. The exception is anaphylaxis, where the reaction involves the whole body: swelling of the lips, tongue or throat, difficulty breathing, wheeze, a sudden fall in blood pressure, faintness or collapse. It can develop within minutes, and the common triggers are foods, insect stings and medicines.

Anaphylaxis is a medical emergency. The treatment is adrenaline, given by intramuscular injection into the outer thigh and given early, followed by an ambulance — 999 in the UK — even where the adrenaline appears to have worked, because symptoms can return. Antihistamines and steroids do not treat anaphylaxis and must not be relied on in place of adrenaline.

Allergy, intolerance and sensitivity

These are used interchangeably in ordinary speech and mean different things, which causes a good deal of confusion and some unnecessary dieting.

Allergy involves the immune system. Reactions are typically fast, can be triggered by a tiny quantity, and may be severe.

Intolerance does not involve the immune system. Lactose intolerance, the common example, is a shortage of the enzyme that digests milk sugar. It causes real and unpleasant symptoms — bloating, wind, pain, diarrhoea — but it is dose-related, so a small amount is often tolerated, and it is not dangerous.

The practical difference matters. Someone with lactose intolerance can usually find a workable level. Someone with milk allergy cannot, and needs to avoid it properly. Treating one as the other, in either direction, causes trouble.

Reducing the risk in babies

Advice here has reversed, and anything written before roughly 2015 is likely to say the opposite of current UK guidance.

The old approach was to delay allergenic foods, on the reasonable-sounding theory that avoiding them avoided the allergy. The evidence went the other way: delay appears to increase risk rather than reduce it. Current BSACI guidance is to start solids at around six months and not before four, and to include the allergenic foods — egg, peanut, cow's milk, wheat, fish, tree nuts — where they are part of the family's diet, introducing them one at a time and aiming to have done so before the first birthday. Egg should be cooked. Nuts go in as a smooth butter or ground, never whole, because of the choking risk.

Higher-risk infants are those with eczema, particularly early or moderate to severe eczema, and those who already have a food allergy. A sibling with a food allergy is not on its own a reason to treat an infant as higher risk. For higher-risk infants the guidance is to consider starting from four months, with cooked egg first and then peanut, and to seek advice before doing so.

On breastfeeding, the guidance is more measured than older writing suggested. Exclusive breastfeeding for about the first six months and continued breastfeeding through the first year alongside solids are both recommended, and continued breastfeeding may support a baby's developing tolerance of new foods. It is not, however, established that breastfeeding prevents food allergy. Where breast milk is not available, a standard cow's milk formula is the right choice rather than a “low allergy” one, unless the infant has a diagnosed cow's milk allergy.

What can be done about it

A great deal, though cure is rare. Reducing exposure helps to very different degrees depending on the allergen; the treatments range from antihistamines and steroid nasal sprays to immunotherapy, which is the one approach that changes the allergy itself rather than managing its symptoms.

A note on a phrase worth avoiding. Allergy is not a weak immune system and is not improved by “boosting” or “building up” immunity — the problem is a response that is too vigorous and aimed at the wrong target. What is wanted is accuracy, not strength.