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MIGHT treatment of allergic diseases such as asthma and hay fever have long-term harmful effects? Some biologists in the US believe this could be possible, as Professors Randolph Nesse and George Williams explain in The Sciences, the journal of the New York Academy of Sciences.

Allergic diseases are certainly more common now than they were in the first half of this century. As many as one in seven schoolchildren has attacks of asthma; though half may "grow out" of it, this still means that one adult in 15 is a lifelong sufferer. Hay fever is even more common, though again many people find their symptoms improve after the age of 25-30.

These facts are undisputed, but the experts disagree on the possible causes of the epidemic of allergies. Some blame air pollution, but studies comparing polluted and less polluted cities have shown no convincing differences. Others blame central heating and air-conditioning, which create an enclosed environment with the windows sealed against fresh air. Neither theory explains the high frequency of nasal allergy to pollen - hay fever - which seems to have been virtually unknown until the mid-19th century, when millions of people left the land to live in industrial cities.

Nesse and Williams have looked at the puzzle from another angle: what benefit does the body get from allergic reactions? They argue that many non-allergic symptoms, such as cough and fever, provide useful defences against infections. What, if any, are the benefits of allergic responses?

People who suffer from allergic disorders have an oversensitive immune system that produces large amounts of one type of antibody, immune globulin E. This is vital for defence against parasitic infestations with worms, flukes and fleas, but it also reacts against inhaled dusts - pollens, animal fur and house-dust mites. Nesse and Williams quote work by another American biologist, Margie Profet, who says these reactions are a protection against inhaled and swallowed environmental toxins. People with strong allergic responses get rid of these toxins more quickly: their tears wash the chemicals out of the eyes and nose; sneezing and coughing rid the respiratory system of inhaled toxins. There is some evidence to suggest that people who have lifelong allergies are less likely than the rest of us to develop cancers - at least of the body systems affected by the allergy. Suppressing allergic responses may allow toxins more time to damage health.

Nesse and Williams accept that their arguments have defects, but the underlying question is a serious one: should people be given long-term treatment for patterns of disease which are poorly understood? A better strategy might be to identity the environmental toxins against which the body reacts in asthma and hay fever, and devise treatments to neutralise them.

Unfortunately, attempts to treat allergic disorders by desensitising people against specific triggers have had disappointing results. The breakthrough that is needed is clear identification of the cause of the rising tide of allergic illness.