There is good evidence to show that exposure to pathogens in dirt may train the immune system and reduce the incidence of allergy in later life. So the Western obsession with hygiene may actually be distorting the immune response and causing the increase in the incidence of allergy.

Many people would like to blame the increase in allergy to increasing air pollution from cars and factories. But the link between allergy and the home environment is thought to be equally important. We spend an average of 75 percent of our time at home, and one of the most important sources of allergens in the world is the house dust mite. The warm, humid environment provided by most affluent homes offers these creatures an ideal living place. An article in the magazine New Scientist talks about "humid, hygienic homes".

Differences in exposure may also account for differences in allergy. For example, in affluent parts of the USA, house dust mites and cat fur are the main allergens, while in poorer areas the house dust mite and the cockroach are the main offenders.

Peanut allergy is another case in point. In the affluent USA, the average American eats about five kilograms of peanuts each year, with about 80 percent of children being exposed to peanuts in their first year of life. This may mean that the increasing reports of peanut allergy are simply due to increased exposure to peanuts, and parents of children who are atopic are advised not to expose them to peanuts when young.

Is there some advantage to allergy?

The genes for allergy are common. Why is this? Did they provide the carrier with an important selective advantage in the past? A study from Venezuela suggests that this may be the case.

The parasitic worm Ascaris lumbricoides is a common parasite around the world. Two groups of Venezualan children from the same background were recently examined for the presence of worm infestation. Each group had a different prevalence of allergic diseases. The children who had the most evidence of atopy, and so allergic diseases, had the lowest levels of infestation with the parasite. So it may be that being allergic could be an advantage under certain circumstances.

Can allergy be prevented?

We know that our immune systems undergo a certain amount of programming in the womb and in early infancy. It is only recently, however, that the possible importance of the environment before birth has been examined. Following birth, the immune system continues to develop and the symptoms of allergy vary with age.

Interesting information has come from studies in Germany. Since the fall of the Berlin Wall in 1989, those in East Germany have started to experience Western living conditions. This has been paralleled by a rise in the incidence of allergic diseases. Studies have been carried out on children who lived in the former socialist system until they were three. Between 1991-1992 and 1995-1996 there was a significant increase in the prevalence of hay fever in these children, but no corresponding increase in the incidence of asthma. This suggests that there are important differences in the childhood development of hay fever compared with asthma.

It appears that what happens before the age of three, perhaps including family size, day-care facilities and socioeconomic status play an important role in the development of asthma. Viral infection may be a common link.

Younger children in large families, and children who attend day-care, tend to catch more colds and other viruses early in their lives. These children are less likely to develop asthma. It seems that viral infection "primes" the immune system to a particular type of response early in life which decreases the risk of developing asthma later in life.

It is becoming increasingly evident that allergies are associated with a certain amount of affluence, and certainly a Western lifestyle. It is also becoming clear that protecting our children from dirt and simple viral infections may be doing them more harm than good.

Your grandmother may have told you that "a little bit of dirt never hurt anyone," and she may well have been correct!

If you have a family history of allergy, then your children may well develop allergic diseases whatever you do. But, in all cases, you can almost certainly help their immune systems to develop well by not worrying about their exposure to the general dirt and dust of daily living, and certainly don't worry if they have the normal number of viral infections which every child in contact with the outside world gets each year.

If you are already an allergic person, then all this is too late for you, but there may be some comfort in the thought that your genes for atopy may be protecting you against harmful parasitic infections.

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Eczema - that itchy feeling

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