THE ALLERGY EPIDEMIC

Myths about allergies abound. Allergies are psychosomatic. Being too clean is causing allergies. Honey can cure hay fever… None of these statements is true. What is true is that possibly a quarter of us are alicted by some form of allergy – and that number is on the rise. You might even be a ected by one without realising. So how can we best deal with the allergy explosion?

WHAT ARE ALLERGIES ANYWAY?

If your summer months are blighted by congestion, sneezing and a runny nose, you might think your immune system has gone into overdrive, or that it is especially good at its job. But unfortunately it’s not that simple.

Allergies are caused by the immune system mistakenly reacting to certain innocuous molecules from the outside world. These can be part of anything from cat skin to certain foods. Any molecule capable of doing this is called an allergen. Although allergens pose no real danger to our bodies, their structures are recognised as a threat by some people’s antibodies – immune proteins on the lookout for harmful invaders.

Allergies involve a special class of antibodies called immunoglobulin E (IgE). Different IgE antibodies detect different allergens. When this happens, the antibodies trigger immune cells to release histamine and other inflammatory chemicals, leading to those nasty symptoms, which under normal circumstances would be a useful defence against invading organisms.

This is a highly primed defence mechanism. IgE antibodies bind to immune cells about 1000 times more tightly than any other class of antibody. This means they are usually already attached to an immune cell, and the whole system is ready to respond as soon as an allergen is detected. This happens in seconds to minutes, while other branches of the immune system take days to respond to a cold virus, for example. “It really is a remarkable system,” says Brian Sutton of King’s College London.

One explanation for the sensitivity of the IgE arm of our immune system is that it evolved to detect and eject what was once a common, highly aggressive threat: large, invading organisms burrowing into our skin, airways or guts. In the absence of such parasites in modern, Western lifestyles, the IgE system seems to have begun misfiring, targeting harmless chemical structures instead.

The system is more amped up in some people than others. “I’m not aware that I’m allergic to anything,” says Sutton, so the understanding is that he has low levels of IgE. “But some people will have 10 times, even 1000 times the level of IgE that I have,” he says. How much you have is partly down to genetics.

It is possible that people who are more prone to allergies would have been better at detecting and ejecting parasites back in our evolutionary past.

But being particularly prone to allergies doesn’t mean your immune system is powerful or overly active in general. You are just as likely to succumb to infections as others and there is no strong evidence that allergies make you more likely to develop autoimmune disorders, in which the immune system starts attacking the body.

With some schools and airlines banning nuts, it might feel like we are entering an age of allergy hysteria. But it’s not just a symptom of helicopter parenting or heightened vigilance – allergies really are on the up.

Hay fever, which is pollen allergy, was first described in 1870, but allergies were rare until the mid-20th century. The first rise was seen in childhood asthma, in which allergens or irritants cause the airways to tighten and become inflamed. From 1960 to 1990, childhood asthma rose to epidemic proportions in developed nations, followed by an increase in hay fever. By 1991, the number of family doctor consultations for asthma in the UK had quadrupled in only 20 years, and appointments for conditions such as pollen and dust allergy more than doubled.

This rise in respiratory allergies was followed by a second tide. As asthma and hay fever began to stabilise in the 1990s and 2000s, food allergies in children shot up, increasing by 50 per cent in the US in just 12 years. “We believe asthma and allergies now affect about 25 per cent of the population,” says Syed Hasan Arshad at the University of Southampton, UK.

Prior to 1970, around 10 per cent of people were thought to experience hay fever in rich nations. Studies now suggest it affects as many as 30 per cent of adults and 40 per cent of children. Around 7 per cent of children in the UK and similarly developed countries now have food allergies.

It is tempting to think that these rises might be down to increased awareness and people going to see their doctor about these problems. But studies comparing different generations show this is not the case. And peanut allergies, for example, can be so severe it seems unlikely that doctors simply failed to notice them in the past.

In fact, many more people may be living with allergies without realising. One study of a group of teenagers and adults in Copenhagen, Denmark, found that half the asthmatics in the group were undiagnosed and not receiving treatment. Rhinitis – hay fever-like symptoms that can also be triggered by dust mites, pets and mould – had not been diagnosed in a third of those who had it.

Rapidly developing countries are now beginning to show similar increases in allergies, particularly China. Between 1990 and 2011, asthma rates in Shanghai among 3 to 7-year-olds rose from 2 to 10 per cent. We still don’t know what has caused this rise in allergies, but there are clues. Most obvious is how much Western lifestyles changed in the 20th century.

Cleanliness is next to godliness, as they say, so for those who are a little less fastidious, the idea that dirt could protect from allergies might have a certain appeal.

First proposed in 1989 by epidemiologist David Strachan, the thinking behind this “hygiene hypothesis” was that modern life has become more hygienic, leading children to catch fewer infections. This somehow predisposes them to develop allergies, perhaps because their immune systems have been incorrectly trained. If so, allergies are the price people in developed nations pay for massively reduced infant mortality.

It is an idea that caught hold with the public, but it doesn’t fully add up. We now know that childhood infections don’t seem to make you any less likely to develop allergies. And major cities like London and New York had largely cleaned up their acts by 1920 – long before the idea was put forward. Water chlorination and separate sewage systems made cholera and typhoid infections rare. And if clean living is to blame for allergies, it doesn’t make sense that it took 40 years for asthma to begin to rise. Since 1960, developed countries have seen only minor changes in hygiene, so what prompted the sudden and recent surge in food allergies?

What we do know is that older siblings are more likely to get some allergies than younger siblings, and that children who grow up in big families or on farms are less likely to develop them. Rather than cleanliness in the home or catching infections, the key factors seem to be spending time with other children and being outdoors in early life – probably because this maximises the range of microbes we meet as infants. As people in the West moved towards spending more of life indoors and in urbanised environments, as is now happening in nations like China, they may have lost contact with particular microbes that have helped hone the human immune system for millennia. This is known as the “old friends” hypothesis.

How such friendly microbes may help prevent our IgE system from misfiring is still largely unclear. But we are beginning to understand that the different bugs living in our bodies can affect many aspects of our health. It is possible that some of them help quieten down elements of the immune system, preventing any overreaction. You are unlikely to restore these missing microbes simply by neglecting your personal hygiene, however. And by the time you are an adult, microbiome – it is probably too late to up sticks and move to a farm.

CAN EXPOSURE CURE MY ALLERGY?

If you have ever heard someone claim they have cured their hay fever by spending time outdoors or that a daily serving of honey – which contains pollen grains – has built up their immunity, don’t fall for it. “There’s not much evidence for that,” says Arshad.

Similarly, forcing yourself to eat peanuts if you are allergic to them is a bad idea, as is living with a pet in the hope that one day you will get along fine.

The idea is grounded in sensible science, though. Clinical injections of small, increasing doses of an allergen can desensitise the immune system to some allergies. When it is administered over several years, there is good evidence that this treatment can work for allergies such as bee and wasp stings, as well as nasal inflammation caused by grass pollen, tree pollen or house dust mites, says Graham Roberts of the University of Southampton, UK. The treatment, often known as allergen immunotherapy or “allergy shots”, seems to gradually build up the amount of exposure the immune system can handle. But as the technique can prompt swelling or itching near the injection site, and in some cases anaphylaxis, breathing problems and collapse, it should be administered by a clinician at a hospital.

The treatment, often known as allergenimmunotherapies have had good results in children, providing lasting effects, but patients need to keep eating peanuts to maintain the protection, says Roberts. Immunotherapy seems to work well for cat allergies too, but is less effective for dog allergies.

However, this isn’t something you can mimic yourself by frolicking in the grass or buying a cat – in fact, doing so is likely to make the problem worse.

ARE WOMEN MORE ALLERGY-PRONE?

In adult life, women are certainly more likely than men to report having allergies and intolerances to food. Intolerances differ from allergies, but they are often grouped together in studies. Last year, a paper examining 2.7 million health records found that 4.2 per cent of women have food allergies or intolerances, compared with 2.9 per cent of men. And this isn’t just a reflection of the apparent rise in gluten intolerance: the most common problem food for both men and women was shellfish, followed by certain fruits and vegetables.

It is possible that women simply pay more attention to what they eat and their health, so are more likely to notice if they have a reaction, but biological factors seem to be involved too. For example, men have higher levels of a certain type of antibody thought to help block allergic reactions.

And hormones almost certainly play a role because the gender difference in allergies switches at adolescence. Before puberty, boys are affected by allergies twice as much as girls.

CAN YOU GROW INTO AND OUT OF ALLERGIES?

Adult-onset hay fever often comes as a surprise to those newly affected – but it is true, you really can develop fresh allergies throughout your life. Just because you have never been allergic to pollen or peanuts, doesn’t mean you never will be. The flip side is that allergies can sometimes fade away, although this is rarer once you have entered adulthood.

You are most likely to grow out of an allergy as a child. One study of more than 40,000 children in the US found that around 26 per cent outgrew their food allergies, usually by the age of 6. Egg, milk and soy allergies are those most often left behind. About 20 per cent of children lost their peanut allergies. When young children have eczema – a skin condition linked to allergies – this often improves in late childhood only for it to be replaced by asthma. The same children are then likely to get hay fever as teenagers, which tends to subside around their mid-20s. This procession of changing allergies is known as allergic march. Some symptoms from the old allergies tend to continue at some level. “It often doesn’t go away, it improves,” says Arshad.

The reason for this progression is still a mystery. “I wish I knew why it changes,” he says. “If I did, I could find a cure for allergies.”

Drugs, hormones, other medical conditions and exposure to smoke and other air pollution may all play a role in changing allergies, says Neil Kao, of the Allergic Disease and Asthma Center, in South Carolina. After the menopause, women no longer experience allergies at a higher rate than men, he says.

But if you think you have recently beaten an allergy, the chances are you have just been exposed to less of the allergen responsible. “Geographically moving is the single most common reason why people think they’ve outgrown their allergies,” says Kao.

What of the oft-cited idea that allergies change every seven years? It is really a lot less predictable than that. “Every person’s immune tolerance levels fluctuate throughout his or her life, spontaneously and naturally. This is dictated by their genes,” says Kao.

DO ALLERGIES COME IN GROUPS?

If you know someone who can’t eat shellfish and also complains about dust, they aren’t (necessarily) just being picky. Allergies can come in a gang, and some combinations are more common than others.

This is because allergies are caused by IgE antibodies recognising harmless molecules as a threat, and allergens with a similar structure can trigger responses in the same person.

This often occurs in two phases. Typically, someone who developed an allergy to a common allergen such as pollen as a child will begin reacting to similar chemical structures in food later in life. This is called cross-reactivity and can lead to oral allergy syndrome: the rapid onset of local symptoms such as itching, tingling and swelling after eating certain foods. As many as 19 per cent of people may have it. “It’s a fairly common type of allergy that isn’t dangerous, but it can be quite annoying,” says Arshad.

Tree pollen can cause allergies to a range of fruits, nuts and vegetables, such as apples, almonds and carrots. Ragweed allergies are associated with problems with melons, bananas, tomatoes and cucumbers, while mugwort pollen can trigger allergies to herbs and spices such as coriander, parsley, cumin and fennel seed. Many other groupings occur, including latex-related allergies to bananas, apples, potatoes and tomatoes.

CAN I PREVENT MY CHILD FROM DEVELOPING ALLERGIES?

It was gospel medical advice for years. Women were urged to avoid commonly allergenic foods such as peanuts during pregnancy and breastfeeding, and were told that their children shouldn’t eat these foods during their first few years of life. The advice was designed to prevent kids from getting allergies. But in the past few years, we have discovered that, in most cases, this was wrong.

Whether women eat allergenic foods during pregnancy turns out to make no difference, and they are no longer advised to avoid them.

But certain things do seem to have an effect. A study of nearly 6000 infants last year found that being exclusively breastfed during the first four months of life seems to be linked to a reduced chance of developing hay fever in low-risk children.

But breastfeeding isn’t the whole story. Evidence is growing that, instead of avoiding allergenic foods, it may be important for high-risk children – those with moderate or severe eczema, or those whose parents or siblings have allergies – to encounter them early on. Those that are introduced to peanuts between the ages of 4 months and 11 months are 81 per cent less likely to develop a peanut allergy by the time they are 5. Similarly, children who begin eating eggs at the age of 4 to 6 months are less likely to develop egg allergies.

ome guidelines in the US and Australia now recommend slowly and carefully introducing infants to potentially allergenic ingredients from the age of 4 to 6 months, although the World Health Organization still recommends giving babies nothing but breast milk until they are 6 months old.

The key appears to be exposing a baby’s immune system to potential allergens before they have a chance to develop allergies – a narrow window of opportunity.

But how this works is still unclear and in many cases the evidence is contradictory. Eating peanuts early in life may be beneficial, but growing up in a home where a lot of peanuts are eaten seems to make peanut allergies more likely.

The idea that a lack of “good” microbes in the gut may be to blame for rising allergies has prompted interest in using prebiotics to change the gut flora of a mother or her infant in order to protect against allergies. However, there is no good evidence yet that these work.

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THE MOST COMMON ALLERGIES

Food
There are many kinds of food allergies, but milk and egg allergies are the most prevalent type among young children in the US. People can also be allergic to peanuts, tree nuts, soy, fish or shellfish. Some are allergic to wheat, but there is no such thing as a gluten allergy. Coeliac disease, an autoimmune disorder triggered by the gluten found in cereal grains, is not an allergy.

Insects
Stinging insects such as bees, wasps and hornets inject venom that most people quickly recover from, but it can provoke life-threatening allergic reactions in some.

Pollen
Hay fever, or seasonal allergic rhinitis, is one of the most widespread allergies. It is usually caused by plants that don’t use animal pollinators to reproduce, instead releasing large amounts of pollen into the air.

Pets
Allergies to furry animals are especially common among people who have other allergies or asthma. Pet hair itself is not an allergen, but it can carry urine, saliva and dander (flakes of dead skin), all of which can provoke allergic reactions. No cat or dog is truly hypoallergenic.

Household pests
House dust mites and cockroaches produce waste that can trigger asthma and allergies. These pests are thought to be two of the most common causes of year-round allergic symptoms.

Latex
Around 1 per cent of people in the US experience an allergic response to the latex protein. Healthcare workers and people who had a lot of medical care as children are particularly susceptible.

Mould
There are many types of fungal moulds, but only a handful cause allergic reactions. When mould spores get into the nose and lungs, they can cause hay fever-like symptoms and asthma.

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FACT ALCOHOL CAN WORSEN YOUR ALLERGIES

Alcoholic drinks can contain histamine, a chemical released as part of the body’s immune response. This can amplify allergic reactions and boost hay fever symptoms, particularly in women.

Written by Penny Sarchet in "New Scientist Magazine",UK, August 11 2018,vol.239, n.3190, excerpts pp.29-33. Digitized, adapted and illustrated to be posted by Leopoldo Costa.