Early exposure to other children lowers adult risk of hay fever but increases risk of asthma

Children who live with several siblings or who go to nurseries have less hay fever, but more asthma as adults, suggests a large international study in Thorax.

The findings are based on interviews with over 18,500 adults aged 20 to 44 from 36 countries in Europe, the USA, Australia and New Zealand. Blood samples were also taken from over 13,000 to measure levels of IgE, antibodies involved in the response to allergens such as house dust mite, cat, and grass.
Family size, access to nurseries, and household income varied across the countries surveyed, but the critical factor seemed to be the degree of exposure to other children in early childhood which predicted the likelihood of hay fever and asthma as an adult.

Singletons regularly exposed to other children at nurseries, had less hay fever as adults. This fits with the “hygiene hypothesis,” which holds that the bacterial assault from infections programmes the immune system not to respond to allergens.

Children with brothers and sisters also had less hay fever, but only if they had detectable IgE in their blood as adults or they had parents who were allergic. And the more sibs they had, the lower was their risk, suggesting that exposure to other children is protective for those who are already “sensitised,” say the authors.

But asthma symptoms were more common in children who had been to nursery before they were 5 years old. And having more than two sibs significantly increased the risk of adult asthma, despite reducing the risk of hay fever. The effects seemed to be strongest among those who did not have allergic (atopic) asthma.

The authors conclude that the bacterial infections acquired from other children do indeed seem to protect against hay fever, but not necessarily against asthma, suggesting that bacterial respiratory infections in early childhood may irreversibly damage the lungs.

Their findings also suggest that IgE may not be the only culprit in allergic sensitisation, they say.