Scientists say they are uncovering why some people's asthma is triggered by shellfish, insects and parasites.
The clue lies in a compound called chitin that gives beetles and crabs their protective outer shell and invertebrates their internal structure.
Humans and other mammals lack chitin, and have developed defences against it.
The latest work in Nature builds on previous animal studies showing a link between asthma symptoms and immune reactions to chitin.
Common trigger
The team at the University of California, San Francisco, found chitin triggered an allergic inflammatory response in the lungs of mice and increased production of a chitin-destroying enzyme, called chitinase, made by cells lining the lung airways.
In humans, individuals have different versions due to small genetic differences.
The researchers are now trying to determine if some of the variants are less effective at breaking down chitin, and if people with those variants are more prone to asthma because they are less able to control their exposure to inhaled chitin.
"This study reveals a mechanism by which chitin can trigger allergic inflammation"
Insects, moulds and parasitic worms produce billions of tons of chitin each year, making it one of life's most common compounds.
Asthma experts have already noticed that some professions - including people working in the shellfish processing industry - have higher rates of asthma.
Lead researcher Dr Richard Locksley said: "It is possible that afflicted workers have forms of the chitinase that functions less well than the other common genetic variants.
"These are areas we are interested in following up."
Leanne Male, Asthma UK's assistant director of research, said: "This study reveals a mechanism by which chitin can trigger allergic inflammation."
She said more work was needed to confirm the findings in humans and establish how this mechanism might apply to everyday situations.
She said 5.2million people in the UK have asthma and are continuously exposed to chitin present in the environment.
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