The “hygiene hypothesis” suggests that exposure to different types of microbes may protect against the development of diseases caused by allergens, but a new study in mice from Cornell University reveals that exposure to a variety of microbes and allergens in adults may actually worsen certain allergic symptoms.
Professor of Immunology Avery August said: “Our data suggest that it is important to think about how we move around in the world and protect ourselves from exposure to microorganisms, because depending on our condition, if we move from a clean environment to a dirty environment, or from a dirty environment to a clean environment, we may respond differently in terms of developing allergic disease.” August is senior author of the study published in the Journal of Immunology.
According to the paper, the “hygiene hypothesis” postulates that exposure to a variety of microorganisms can protect against allergic-type diseases. For example, this hypothesis suggests that growing up on a farm or in a less clean environment may protect against allergic reactions. Published epidemiological and experimental data strongly support this hypothesis. However, the current study found that such protection may be nuanced and dependent on life stage and timing of exposure.
A key question is where is the dividing line between when exposure to a wide variety of antigens is protective and when it can be exacerbating? ”
Avery August, Professor of Immunology, Cornell University
In this study, researchers exposed mice to airway exposure to dust mites and examined whether they developed allergic airway inflammation. This model is the one used in mice to study asthma in humans.
The researchers used so-called “specific pathogen-free” mice. These are mice raised in a clean environment with low exposure to a wide variety of microorganisms and free of specific disease-causing pathogens. In this way, researchers can ensure that infections do not interfere with their experiments.
These clean mice were then co-housed in the same environment as commercially available mice exposed to a variety of microorganisms and shared with pathogen-free mice due to their close proximity. Next, we also exposed adult and neonatal mice, which do not carry the specific pathogen, to a dust mite allergen to determine whether allergic airway inflammation would develop.
Researchers found that exposure to microorganisms as an adult exacerbated the development of allergic airway inflammation compared to newborns exposed to these microorganisms.
In next steps, the researchers hope to test the timing as mice age to see when protection from microbial exposure wanes, whether certain types of microbes play a role in disease development, and how the effects found in this study apply to other types of disease.
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Reference magazines:
Elmore, J. others. (2026) Exposure to diverse microorganisms exacerbates the development of allergic airway inflammation in adult mice. Journal of Immunology. DOI: 10.1093/jimmun/vkaf331. https://academic.oup.com/jimmunol/article/215/2/vkaf331/8494900
