A childhood spent on the farm may provide lifelong protection against certain allergies including hay fever.

Researchers found that adults who had grown up on a farm were less likely to develop allergies to cat dander, certain types of grass and pollen, compared with adults who did not spend their childhood on farms. Farm children were also somewhat less likely to develop allergies to dust mites.

But there was no association between living on a farm as a child and having asthma or wheezing symptoms as an adult. Previous studies have suggested that growing up on a farm may lower the risk of asthma.

Still, the findings indicate that exposure to certain environmental factors as a child can provide lifelong protection against certain allergies.

At this point, it is unclear how early life exposure to a host of potential allergens may reduce the likelihood of developing allergies and asthma later in life. Health experts speculate that the children's early exposure to allergens and infections primes their immune systems to resist them later on.

Studies have also found that young children exposed to older siblings at home and those who attend day care also have a lower risk of allergies and asthma.

All of these findings are in step with the so-called "hygiene hypothesis" -- the theory that a lower exposure to germs and other environmental microorganisms affects the immune system's development in such a way that it is more prone to allergic reactions.

American Journal of Respiratory and Critical Care Medicine 2001;164:1829-1834