Smog, traffic, crime, and drugs--the hazards of city life make the grass seem greener in the country. Now a new study provides support for the idea that growing up on a farm may also help people breathe easier. The study, reported in the 10 August issue of The Lancet, shows how early exposure to microbes on the farm can alter the immune system in a way that might prevent childhood allergies and asthma.
Asthma and respiratory allergies have been rising for decades in developed countries. Backers of the so-called hygiene hypothesis suggest that it's because we're too clean for our own good. Early exposure to bacteria and viruses, they say, trains the immune system to fight off invaders, but a spotless environment leaves immune cells unemployed and spoiling for trouble. They then overreact to harmless substances such as cat dander and pollen. Kids in rural towns, in fact, are five times more likely to develop allergies or asthma than children raised on a farm.
One reason may be exposure to bacteria, which are more prevalent in farm houses than other homes. Test-tube studies show that high levels of bacterial debris spur certain immune cells to kick-start a line of immune defense called the innate response, which fights microbes indiscriminately. To see whether that happens in people, a team led by pediatric immunologist Roger Lauener of the Zürich University Children's Hospital in Switzerland took blood samples from 25 children from small alpine dairy farms and 71 children from rural villages, and then compared the activity of genes encoding receptors involved in innate immunity.
The genes for two of those receptors, called CD14 and Toll-like receptor 2, were more than twice as active in farmers' kids as in the city dwellers. That suggested that the innate immune response could play a key role in allergy and asthma, Lauener says. The next step, he adds, will be to compare children from allergy-prone and healthy families to find out the relative importance of nature and nurture in allergies and asthma.
Immunologist Xi Yang of the University of Manitoba in Canada says that data fall short of implicating the innate immune response. But pediatric immunologist Andy Liu of the National Jewish Medical and Research Center in Denver, Colorado, says it does "substantiate their story" that farm children develop fewer allergies because of microbial exposure. It's way too soon to recommend that parents haul their tikes to the farm to prevent allergies, he says, but "maybe we're onto something promising for reversing this global trend."