Comparing dirty, wild rats and mice to their squeaky-clean laboratory-bred counterparts may help explain why people who live in industrialized societies have higher rates of allergy and autoimmune diseases than those who live in less developed areas.
A new study comparing the rodents provides new evidence that supports this “hygiene hypothesis”, which holds that people who live in “hygienic” societies have higher rates of allergy and possibly autoimmune diseases because their immune systems have not been as challenged by everyday germs and microbes.
The study showed that wild rats and mice had higher levels of two types of antibodies normally associated with triggering allergies and autoimmune diseases in laboratory animals. But in the wild, these antibodies appeared to have a protective effect and defend the animals against potential threats.
Researchers say up to 50 million Americans suffer from allergies and another 8 million have some form of autoimmune disorder, such as lupus, type 1 diabetes, or rheumatoid arthritis. Autoimmune disorders occur when an overactive immune system attacks tissues in the body.
Rats Support Hygiene Hypothesis
Although the hygiene hypothesis has been widely accepted, researchers say it has not been thoroughly tested in animal studies.
"Laboratory rodents live in a virtually germ- and parasite-free environment, and they receive extensive medical care -- conditions that are comparable to what humans living in westernized, hygienic societies experience," says researcher William Parker, PhD, an assistant professor of experimental surgery at Duke University Medical Center, in a news release. "On the other hand, rodents living in the wild are exposed to a wide variety of microbes and parasites, much like humans living in societies without modern health care and where hygiene is harder to maintain."
In the study, published in the Scandinavian Journal of Immunology, researchers compared levels of various antibodies associated with allergies and autoimmune diseases known as immunoglobulins (Ig) in trapped wild rats and mice and those bred in Duke University’s animal facilities. When an animal’s immune system encounters a foreign invader, it responds by producing these antibodies, which bind to the invader and destroy it.
The results showed that the wild rodents had higher levels of two classes of immunoglobulins, IgG and IgE. IgG is often involved in autoimmune disease, and IgE is often implicated in human allergic reactions and plays an important role in fighting against parasites.
Researchers say they expected the wild rodents to have higher levels of IgE because they would have encountered more threats in the wild. But the production of an unusual type of IgG was unexpected.
This polyreactive, autoreactive type of IgG is usually associated with another class of antibodies and it appeared to have a beneficial effect on the wild rodents.
"These results appear to demonstrate that the environment has profound effects on the production of IgE and autoreactive IgG," says Parker. "While the production of these two antibody types lead to autoimmune disease and allergy, respectively, in the laboratory animals, their production seemed to represent a nonpathogenic, protective response to the environment by the wild rodents."
By Jennifer Warner, reviewed by Louise Chang, MD
SOURCES: Devalapalli, A. Scandinavian Journal of Immunology, 2006 online early edition. News release, Duke University Medical Center.