Why are we becoming allergic to modern life?

The number of allergy sufferers is increasing, despite high standards of hygiene. Are our immune systems failing us because we are too clean for our own good?

For years the 50-year-old woman had boiled Swiss chard at least once a week for her family. But one weekend she was cleaning and cooking the shiny green leaves of the vegetable when she suddenly and unexpectedly suffered an asthmatic attack caused by inhaling the vapour from the saucepan.

For years the 50-year-old woman had boiled Swiss chard at least once a week for her family. But one weekend she was cleaning and cooking the shiny green leaves of the vegetable when she suddenly and unexpectedly suffered an asthmatic attack caused by inhaling the vapour from the saucepan.

Doctors who examined the woman and tested her reactions to a number of compounds found in the vegetable now believe that the cause of her attacks was a tiny amount of a protein called 42 Kda. Although only minute amounts of it are present in Swiss chard, it was enough to trigger a cascade of asthmatic symptoms.

Swiss chard, which has not previously been linked to such an allergic reaction, has now joined a growing list of allergens that range from pollen, dust-mites and boiled spinach, to latex, house mice and fried steak.

Allergic reactions to these, and a myriad of other allergens, are on the increase in both adults and children, with one in four of the population affected at some time in their lives. But just why the number of sufferers is increasing is not clear. One theory - the hygiene hypothesis or rattling-window syndrome - is that modern life is too cocooning and that in the absence of real challenges the immune system starts picking on harmless compounds.

An allergy is usually the result of the immune system making an inappropriate response to a compound that is inhaled, touched, eaten or injected. In most cases the compound is not intrinsically harmful, and it is the immune system response that is the problem. The common symptoms of sneezing, wheezing, runny nose, swellings, itchy eyes, and shortness of breath, are not so much caused by the allergen itself as by the body's attempts to flush it out of the system.

According to data from the British Allergy Foundation, the number of sufferers is going up by 5 per cent every year, and as many as half of them are children.

Some of them are pre-disposed to developing an allergic reaction, largely because they produce more of the allergy antibody known as IgE, a front-line combatant turned out by the immune system to tackle parasitic invaders.

Such pre-disposed people are more at risk of also developing an allergic disorder like asthma, eczema, hay fever, or hives.

But, as the number of sufferers increases, so too has the range of allergens. Doctors in Madrid have found not only the case of the Swiss chard housewife, but also that of a 51-year-old man who was allergic to sirloin steak and other bovine meat.

Why the numbers of allergens and sufferers is increasing is still not known for sure. The finger of blame is increasingly pointing at environmental and societal changes that have occurred in most developed countries since the Sixties. "Behind the hygiene hypothesis is the idea that if the immune system doesn't have any infections or parasitic infestations to cope with, it turns to other substances, resulting in allergies,'' says Dr Lawrence Youlten, consultant allergist at the London Allergy Clinic.

It's argued that the cleaner, more controlled environment that children are now brought up in does not challenge the evolving immune system hard enough. Because children are not exposed to the same infections or bacteria prevalent even half a century ago, it's argued, the immune system is not as honed in as much as it used to be. When the immune system was busy zapping real infections, so the theory goes, it had no time to deal with more minor allergens and irritants. But now many of the old infections are no longer a threat, the immune system focuses on allergens.

"People are now more protected. Your bedroom is a warm, badly ventilated room with carpets and soft furnishings full of house dust-mites. When I was young you wore gloves to keep you warm or you lit a fire. In those days there was certainly less asthma,'' says Dr Youlten.

Support for the hygiene theory comes from a growing number of areas. Research carried out in London, for example, has found that young children in a family are less likely to have allergy problems than their older siblings. It's thought that younger children, who are exposed to bugs brought home by their older siblings, are able to develop an immune system that's less sensitive to allergens. A decline in larger families would explain some of the increase in cases.

Further evidence for the theory comes in three new pieces of research which all show that the children of farmers have lower rates of allergies, hay fever, wheezing and asthma. In all three cases it's suggested that exposure of farm children to bacteria and microbes in infancy had a toughening-up and protective effect on the body.

Although the hygiene effect can explain the rise in cases of allergy, why some people suddenly develop allergies as adults is still a mystery.

"Most people who are in the running for an allergy have it before they are 20, but there are people who can suddenly become allergic," says Dr Youlten. "I had a patient of 70 who developed classical hay fever from grass pollen which he had been exposed to for the previous 70 years without any effect. We simply don't know why this happens."

For years, many allergy sufferers have had to put up with their symptoms or avoid whatever their immune system has latched onto. But immunotherapy is now gaining wider acceptance as a weapon in the fight against allergies. Allergen immunotherapy works like a vaccination. As the body is exposed to small, injected amounts of a particular antigen, it gradually gains tolerance to the allergens. Researchers have found that patients with pollen allergies who went through a complete course of immunotherapy felt the beneficial effects of it for more than three years after treatment had ended.

There are hopes that eventually immunotherapy will be available for an increasing number of antigens. But while the more common triggers are likely to be dealt with in this way, rarer antigens will not. For those with allergies to Swiss chard, sirloin steak, or boiled spinach, the only real solution is likely to be abstinence.

British Allergy Foundation helpline: 0208-303 8583