Allergy

Mama,don't fence me in

Lower incidence of allergies are found in big families
with lots of kids and 'filthy' animals

By Owen Dyer

The May 1 issue of the British Medical Journal (BMJ) will come as a rude shock to all those nervous young mothers who insist that friends wash their hands before picking up their little treasure. A large Danish study suggested that keeping babies in a virtual bubble of sterility is asking for trouble in terms of allergy.

Something in our cosseted modern lifestyle must be driving the phenomenal growth of allergies, because there's no parallel increase in the developing world. Numerous studies have shown that a larger number of siblings, early exposure to animals and avoidance of antibiotics are all protective against the development of atopy.

This latest research in the BMJ actually shows a slight positive association between early infection and atopic dermatitis � at least in girls � and no evidence of a protective effect of clinically apparent early infection in boys. In all other respects, however, it backs up the claims of those who say reasonable exposure to pathogens is generally good for youngsters.

The researchers looked at 24,341 women who gave birth to singleton infants. The mothers were asked about infections, antibiotic use, allergic symptoms and exposure factors both before and after giving birth.

Four principal factors were used to measure exposure to pathogens: high number of siblings, attendance at day care, pet ownership and farm residence. The presence of each of these factors reduced the probability of atopic dermatitis by 10-20%. A child exposed to all four scenarios would be only about half as likely to develop atopic dermatitis by 18 months. Early day care was the biggest single factor in reducing the risk of later dermatitis.

The authors point out that other studies have found higher rates of various infections in people with a preexisting allergy. That suggests that the link seen between childhood infections and later contact dermatitis may be an artifact of weaker immune systems that are perhaps already predisposed to allergy.

In an accompanying editorial, the BMJ's science editor Geoff Watts highlighted a new theory that explains why less exposure to pathogens may appear to cause allergy. The original 'hygiene hypothesis' claimed that reduced exposure led to a low level of activity in T1 lymphocytes, which in turn caused an excess of T2 lymphocyte activity, producing allergy. This theory didn't stand up to the fact that T1-mediated conditions, like type I diabetes and inflammatory bowel disease, are also on the rise.

The latest thinking, said Watts, holds that a third group of immune cells � regulatory T cells � got used to the permanent presence of relatively benign tenants, such as lactobacilli and helminth worms, and so prevent the immune system from overreacting to such harmless infections. Such a scenario would explain why clinically apparent infections are not protective against allergy � it's the bugs that don't get noticed that count. "As the Duchess of Windsor once remarked," said Watts, "one can never be too thin or too rich � wisely, she did not add too clean."