A major function of a healthy immune system is to protect us from things in the environment that may do us harm. In order for it to effectively perform this function, it must first be able to accurately distinguish between what belongs in our bodies from what does not. Then it must mount an appropriate response to malicious invaders, such as harmful bacteria, viruses, or cigarette smoke.

Over the years, researchers have come to believe that like healthy muscles, healthy immune systems perform better when used. The theory, called the hygiene theory, was introduced in 1989 and goes something like this: the infections we encounter in childhood improve the performance of our immune system so we are less likely to develop immunologic problems, like allergies, later in life. To test this hypothesis, a group of researchers set out to see whether children who had fevers (a common sign of infection) during their first year of life were less likely to develop allergies as they grew older. The results of their study were published in the February issue of the Journal of Allergy and Clinical Immunology. The researchers found that not only were children who had fevers early in life less allergy prone, but that the more fevers they had, the more allergy resistant they became.

About the Study

The researchers enrolled 835 newborns in their study. They tracked the clinical records of these children for the first year of their lives, monitoring the number of fevers and respiratory infections reported as well as their level of antibiotic use. Of these initial 835 children, 441 underwent clinical evaluation by an allergist at the age of six or seven. This evaluation included a medical history and physical exam and skin prick and blood tests for sensitivity to allergens, which are substances that cause allergic reactions.

The Findings

The researchers found that among children who had no fevers during their first year, 50% demonstrated allergic sensitivity (either a positive skin prick test or a positive blood allergen test). Among children who had one fever, 46.7% demonstrated allergic sensitivity. Among children who had two or more fevers, on the other hand, only 31.3% demonstrated allergic sensitivity.

After adjusting for a number of variables, the researchers determined that each fever a child has during the first year of life, reduces his or her odds of developing allergies later in life. This was particularly true of children who had fever-inducing upper respiratory infections.

How Does This Affect You?

Past research has shown that children with older siblings are less likely to develop allergies than children with younger siblings or none at all. The assumption was that the older children were inadvertently exposing their younger brothers or sisters to every bug in school. The same idea was also used to explain why children who attended daycare at an early age seemed to have fewer allergies. None of this previous research, however, directly measured the incidence of infection and compared it to the development of allergy resistance. The results of this study offer the most direct support to date of the hygiene hypothesis and may lead to preventive therapies for asthma and allergies.

This study, like other epidemiologic studies, has its limitations. For example, because the study was based on the mothers’ ability to accurately remember the number of fevers their children experienced, and because not all fevers necessarily result in treatment in a clinical setting, certain biases that are avoidable in other types of research (like randomized, controlled trials) may have been introduced.

In the end, proving the hygiene hypothesis, and understanding all it’s implications, will require more research. But in the meantime, the results of this study may help ease the anxiety of stressed out new mothers. It may be that all those colds and runny noses their children are bringing home from daycare, are actually doing them some good.