While there is no definitive evidence for the reason behind the rise in food allergies over the past decade or so, one of the most popular explanations is the "hygiene hypothesis." The main premise of the hygiene hypothesis is that decreased exposure to germs and other disease-causing substances due to some characteristics of modern Western lifestyles has affected the human immune system's opportunity to develop standard immune responses. Because of this lack of opportunity, the immune system becomes prone to respond by reacting to otherwise harmless substances: In other words, by developing allergies.
Researchers came up with the hygiene hypothesis in an attempt to explain the disparity in patterns of atopic (or allergic) disease between North America and Western Europe and some other parts of the world. The hygiene hypothesis is also put forth to explain why children raised on farms or with pets tend to have fewer allergies than children raised in cities or suburbs, or without pets, respectively.
The means by which the hygiene hypothesis might work are still a matter of debate. One difference in areas affected by the hygiene hypothesis may be the presence or absence of certain types of helpful bacteria that normally live in the small intestines; another may be the effects of early exposure to certain infectious diseases; another may be exposure to parasites. Researchers have studied all three of these possibilities in hopes of finding a conclusive cause of the disparities in allergies between people with different lifestyles and in different parts of the world. But thus far, none have been proven.
The hygiene hypothesis is not by any means the only explanation that has been put forth to decipher the recent rise in food allergies. Some researchers speculate that environmental factors (like toxins or chemicals) may play a role. Another possibility is that the diets women eat during pregnancy or breastfeeding may somehow affect their babies' immune systems.
So what are the implications of the hygiene hypothesis for people with food allergies, or people in families with histories of food allergies? At this point, the hygiene hypothesis has more relevance as a direction for research study than as a directive for parents hoping to prevent their children from developing allergies. Basic hygienic standards like hand washing and following food safety practices are proven to reduce the transmission of infectious diseases. Nonetheless, because the hygiene hypothesis is a major area of interest among researchers studying food allergies and because it's mentioned frequently in news pieces about the increased prevalence of food allergies, it's useful for parents be aware of it.
Sources:
Bloomfield, S.F., et al. "Too Clean, Or Not Too Clean: The Hygiene Hypothesis and Home Hygiene." Clinical and Experimental Allergy. Apr. 2006 31(4): 402-25. 22 Oct. 2007.
McGeady, Stephen J. "Immunocompetence and Allergy." Pediatrics. Apr. 2004 113(4): 1107-13. 22 Oct. 2007.
