The researches at Duke University in North Carolina found evidence that may help to explain why people in industrialized societies that greatly stress hygiene have higher rates of allergy and autoimmune diseases than do people in less developed societies in which hygiene is harder to achieve or considered less critical.

The researchers published their results early on-line in the Scandinavian Journal of Immunology. The research was supported by the National Institutes of Health, the Duke University School of Medicine and the Fannie E. Rippel Foundation. 

The prevailing hypothesis concerning the development of allergy and probably autoimmune disease is the "hygiene hypothesis," which states that people in "hygienic" societies have higher rates of allergy and perhaps autoimmune disease because they -- and hence their immune systems -- have not been as challenged during everyday life by the host of microbes commonly found in the environment.

The study suggests that an overly hygienic environment could simultaneously increase the tendency to have allergic reactions and the tendency to acquire autoimmune disease, despite the fact that these two reactions represent two different types of immune responses.

Up to 50 million Americans are estimated to suffer from allergies, and another 8 million have some form of autoimmune disorder, which occurs when an overactive immune system attacks tissues in the body. Examples of autoimmune disorders include lupus, insulin-dependant diabetes, rheumatoid arthritis and scleroderma.

"The most commonly accepted explanation for this high incidence of allergy and perhaps autoimmune disease is the hygiene hypothesis," Parker said. But this hypothesis has not been thoroughly tested in animal studies, he said, and the few studies conducted have focused on specific pathogens or parasites.

The Duke researchers decided to study the hypothesis by comparing the immune systems of wild house mice and common rats to laboratory mice and rats. For their experiments, the researchers trapped wild rats in rural and urban settings in North Carolina and trapped wild mice in Wisconsin. They then measured the levels of antibodies in the blood of the wild rodents and compared the levels to those observed in mice and rats housed in Duke animal facilities.

"These results are consistent with the idea that animals without access to modern medicine have high levels of autoimmune-like and allergic-like immune responses that represent appropriate responses to unknown factors in their environment," he said.

Although this study suggests that the environment plays an important role in how the immune systems in animals develop, genetics is likely to be involved as well, Parker said. He now is planning additional studies to help decipher the full role of genetics.

Also, his group is planning further studies of the hygiene hypothesis, using the new rodent model to examine other factors that may be contributing to the higher rates of allergy and autoimmune diseases of humans in industrialized societies, such as lack of exercise, increased mental stress and the consumption of processed food.

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