Many people around the world are allergic to substances in the environment. But lately, researchers have noted that people in industrialized countries have more allergies and a higher incidence of asthma than do people from less developed countries with lower hygiene standards. Some have suggested that growing up in environments that are 'too clean' can lead to the development of allergies. But researchers aren't sure why this might be.

Richard Locksley from the University of California at San Francisco thinks he might have found a partial explanation for what's being called "the hygiene hypothesis." Locksley took specially bred mice that responded to an array of allergens. He found the mice with strong allergic reactions expressed high levels of chitinase when they were exposed to allergens. Chitinase is an enzyme that breaks down chitin.

Chitin is an important molecule in the exoskeletons of smaller plants and animals - like yeasts and molds, crustaceans - like shrimp and crabs and lobster, and insects - like dust mites and cockroaches that have been implicated in allergy and asthma. Because they don't have an exoskeleton, mammals - like mice and humans - don't have chitin in their bodies. Nonetheless, they have the capacity to break it down using chitinase.

Next, Locksley and his lab assistants had the mice breathe in chitin dust. It caused an allergic reaction in their lungs, and that got Locksley's attention. "It raised the possibility that chitin itself could be part of a trigger that was being responded to," he explains, "and you made the chitinase to break down the chitin associated with these allergen-provoking molecules."

Locksley says many bacteria eat chitin, thereby eliminating it from our surroundings. He says the hygiene hypothesis could be partly explained by having too few bacteria in the environment. "So it's possible that cleaning up the environment and not having lots of bacteria allows chitin from insects and molds and fungi to build up to levels such that they can be aerosolized into lungs at levels that could provoke sustained symptoms."

Locksley says chitin probably can't account for all the allergic reactions in industrialized countries. He says a lot more work needs to be done to find out what role chitin does play in allergy and asthma. His research was published in the online journal Nature.