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Remote control. Yeast infections in the mouse gut lead to allergic reactions in the lungs, filling them with inflammatory cells (arrow), which contain histamine and other proteins (pink).

Is Allergy a Gut Reaction?

When your gut reaction to dust and mold is sneezing and wheezing microbes may be to blame. Studies in mice have now shown that yeast living in the digestive tract can make the immune system much more sensitive to allergens. The work provides experimental evidence for a suspected link between allergies and antibiotic use, and a possible explanation for the increase in allergies seen over the past 40 years, during which time antibiotics have become ever more prescribed.

Perhaps the best known theory to explain why allergies are on the rise says that, because we live in much cleaner homes, developing immune systems never "learn" how to cope properly with pollen, animal dander and other harmless substances. But another hypothesis claims that the increased use of antibiotics plays a role. These drugs could upset the natural balance of the microbial community in the gut, some researchers believe, causing the population of one of the gut's natural inhabitants, the yeast Candida albicans, to explode. The yeast makes a chemical that can modulate immune responses and make the immune system overly sensitive.

To test the hypothesis, immunologist Gary Huffnagle and microbiologist Mairi Noverr from the University of Michigan in Ann Arbor have developed a model in which they treat mice with antibiotics for five days, which rids them of gut bacteria. Then they give the rodents a dose of yeast, mimicking what happens in the human gut when yeast populations surge after antibiotic treatment.

In earlier experiments, the team had shown that these animals' lungs, unlike those of mice that had received neither the antibiotics nor the fungi, produce a moderate allergic reaction to mold. Now, a new study in the January issue of Infection and Immunity shows that the treated mice also develop allergies to another substance, egg white protein. Furthermore, a different mouse strain, treated under the same protocols, reacted similarly, showing that the antibiotic treatment, and not some genetic susceptibility, leads to the allergies, says Huffnagle.

The work provides strong experimental evidence for the antibiotics hypothesis, says Bruce Klein, an infectious disease expert at the University of Wisconsin, Madison. Moreover, the study shows that "the gut could be setting the stage for a how the lung, a distant site, reacts," Klein says. "That's a potentially important observation that needs to studied."

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