A study by researchers at Duke University Medical Center provides more support for the "hygiene hypothesis," the notion that people who live very hygienically are more prone to allergies and autoimmune disease because their immune systems were not exposed to, and therefore not challenged by, common microbes in the environment.
The study compared the immune systems of wild house mice and common rats -- routinely exposed to microbes and parasites -- to laboratory mice and rats, which are raised in a virtually germ-free environment.
The researchers looked at the animals' production of certain antibodies, called immunoglobulins (Ig), which are associated with autoimmune disease or allergy. Antibodies are produced by the immune system to bind to intruders and destroy them.
The wild animals were found to have higher levels of IgG -- involved in autoimmune disease -- and IgE, which fights against parasites and has been linked to allergies in humans, than the lab rodents. The wild rodent also had higher levels of a type of IgG called polyreactive, autoreactive IgG, which has been linked to autoimmune disease in both hygienic humans and rodents but did not appear to be affecting the wild rodents.
The finding show that the environment has profound effects on the production of IgE and autoreactive IgG, researchers said. Because autoreactive IgG is able to bind to environmental antigens in the wild rodents, they don't appear to cause harm.
In hygienic animals, however, autoreactive IgG can bind to the body's own cells, which can lead to autoimmune disease. Similarly, IgE can bind to parasites in wild animals, while in lab animals the antibodies bind to harmless environmental antigens, triggering allergies to those substances.
It's estimated that 50 million Americans have allergies and 8 million have autoimmune disorders, which involve an overactive immune system attacking body tissues.