Exposure to certain animals may prevent rather than trigger asthma and allergies, experts believe.
Handling rodents conferred some protection against asthma
Lab workers who frequently handled research rodents had fewer allergic reactions to the animals as a result, an Imperial College London team found.
The researchers believe it is the type of exposure that is important.
A recent study found early childhood exposure to cats increased eczema risk, whilst early exposure to dogs appeared to be protective.
The latest work, published in the American Journal of Respiratory and Critical Care Medicine, supports the "hygiene hypothesis" of asthma and allergic diseases.
According to the hygiene hypothesis, exposure to naturally occurring infections and microbes might essentially immunise against the development of asthma and allergies.
Dr Meinir Jones and her team suggest that the laboratory workers were experiencing a natural form of immunotherapy via exposure to animals through their occupation.
Dr Jones said: "Interestingly, this does not seem to be the case for other groups at risk of occupational asthma such as bakers and detergent manufacturers."
She believes the difference lies in the fact that lab workers get exposure not only through inhalation but also through the skin if they are bitten or scratched.
Among the 689 lab workers they studied, those who had antibodies in their blood that were specifically produced in response to the rodent allergens had a two-fold reduced risk of developing work-related chest symptoms than those who had only non-specific antibodies in their blood.
Furthermore, the ratios of rodent-specific antibodies were highest in those lab workers who had handled the greatest number of rodents.
Another possibility is that workers handling a larger number of animals might have been exposed to greater levels of dirt and other microbes which may protect against allergic disease, as suggested by the hygiene hypothesis, said the authors.
Commenting on the research Dr Karen Pacheco of the National Jewish Medical and Research Centre in Denver, the US, said: "This approach suggests that peak exposures are more important for the development of immune responses to laboratory animals than average exposures."
A spokeswoman from Asthma UK said although the work was helpful, more research was still needed.
"This research helps us understand the risks people may face when working with animals. However, we still have much to learn about allergen exposure and and the risk of developing asthma."