Peanuts are one of the most common foods that trigger allergies in people, but a new study suggests that it’s not peanuts that are the problem — it’s the dry-roasting process.

Researchers from Oxford University and the University of Pennsylvania injected mice with samples of peanut protein from nuts that were either raw or dry roasted. The mice who got the dry-roasted samples mounted a much more vigorous immune system response than the mice who got raw samples, according to a study published in the Journal of Allergy and Clinical Immunology. An intense immune system reaction is a sign of an allergy.

The researchers followed up by feeding the mice various types of peanut extract. (These extracts were delivered through a tube directly to the stomach.) Again, the mice that had already been exposed to dry-roasted peanut protein had a greater immune response than the mice exposed to raw peanuts. Interestingly, the mice that were initially exposed to proteins from dry-roasted peanuts were more allergic to both kinds of peanuts – raw and dry roasted – than the mice who were primed with protein from raw peanuts.

Finally, the researchers applied extracts from either raw or dry-roasted peanuts to wounds on the animals’ skin. Again, the mice initially exposed to dry-roasted peanuts showed a greater allergic response to eating peanuts – either normally or through a gastric tube – than the mice initially exposed to raw peanuts.

All of the experiments seemed to suggest that something in the dry-roasting process turned essentially harmless peanuts into dangerous, allergy-inducing comestibles. That something is the Maillard reaction, which changes the “physiochemical properties of peanut proteins,” the study authors wrote. They isolated the allergen known as Ara h1 from raw and dry-roasted peanuts and found that the ones in the cooked peanuts had a shape that seemed to trigger the dramatic immune-system response.

The findings could explain why peanut allergies are so much more common in North America and Europe than they are in East Asia.

Though people in both regions have similar levels of food allergies overall, peanuts are a “striking exception” to this trend, the researchers wrote. Peanuts are consumed in similar quantities in both places, but Westerners usually roast their peanuts and East Asians typically eat them raw, boiled or fried, according to the study.