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Large Families May Help Keep Allergies Away

But Early Infections Don't Seem to Be Reason for Protection

WebMD Health News

April 29, 2004 -- Having three or more siblings helps prevent allergies, a new study suggests.

In recent years, researchers have been diving deeper into the "hygiene hypothesis," which aims to explain why some people have allergies and some do not. The thought is that being exposed to more dirt and grime may actually decrease the risk of allergies. It suggests that advances in antibiotics and cleaner homes have contributed to increases in allergy, asthma, and eczema by decreasing rates of childhood infection.

Early studies have suggested that having more siblings may decrease the risk of allergies. But in this new study, researchers sought to determine if this is merely an effect of kids being exposed to more infections from other kids.

Danish researchers studied more than 24,000 mother-child pairs to try to get to the bottom of this dirt and allergy association -- specifically, whether exposure to infections early in life decreases the risk of allergies. The children were enrolled in the study when they were 6 to 18 months old.

The researchers were specifically looking at how many of these children developed eczema, a skin condition that causes a red, itchy rash and occurs in kids prone to allergies. In addition to a host of other questions, the moms were asked about the number of infections their kids had before 6 months of age.

The study appears in the current issue of the British Medical Journal.

Siblings and Pets on the Farm

By 18 months of age, just over 10% of the children had developed eczema. But contrary to the hygiene hypothesis, the researchers found that the more infections the children had before 6 months of age, the more likely they were to have eczema.

But other factors that are associated with increased exposure to bacteria and other infectious organisms, including having three or more siblings, attending day care, having pets, or even living on a farm, decreased the risk of developing eczema.

Since infections did not seem protective, the researchers say there must be some other underlying reason why having more siblings decreases allergies.

But what is it?

Christine Stabell Benn, from Statens Serum Institut in Denmark, and colleagues suggest that having more siblings, being in day care, and the other factors may expose kids to infectious organisms, and thus decrease allergies, without actually causing infection symptoms. This constant exposure may help keep the immune system stimulated to ward off allergies.

Another explanation, they suggest, is that the association between increased infections and more allergies may indicate an immature immune system.

Bring On the Microscopic Worms

Geoff Watts, science editor at BMJ, wrote an editorial accompanying the study suggesting that the tendency to develop allergies may lie in a lack of exposure to organisms found in dirt, such as certain types of microscopic worms, rather than exposure to common childhood infections.

In addition, Watts says that previous research suggesting a lack of certain cells that help regulate the immune system may open the door for new ways to prevent allergies. And he says that early experiments are already under way to see if stimulating these regulatory immune system cells through vaccination may decrease the risk of allergies.

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