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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鈥檚 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 (see 鈥淭he most common allergies鈥�, below). 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鈥檚 antibodies 鈥� 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. 鈥淚t really is a remarkable system,鈥� says Brian Sutton of King鈥檚 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. 鈥淚鈥檓 not aware that I鈥檓 allergic to anything,鈥� says Sutton, so the understanding is that he has low levels of IgE. 鈥淏ut some people will have 10 times, even 1000 times the level of IgE that I have,鈥� 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鈥檛 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.

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Are allergies becoming more common?

With some schools and airlines banning nuts, it might feel like we are entering an age of allergy hysteria. But it鈥檚 not just a symptom of helicopter parenting or heightened vigilance 鈥� 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 , 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. 鈥淲e believe asthma and allergies now affect about 25 per cent of the population,鈥� 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 . 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 鈥� hay fever-like symptoms that can also be triggered by dust mites, pets and mould 鈥� 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 .

We still don鈥檛 know what has caused this rise in allergies, but there are clues (see 鈥�What causes allergies and how to avoid them鈥�). Most obvious is how much Western lifestyles changed in the 20th century.

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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. Actual allergy to gluten is very rare. 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鈥檛 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.

This article appeared in print under the headline 鈥楢llergy explosion鈥�