The microbiomes in our gut and mouth may determine whether people with a peanut allergy develop a life-threatening reaction. This could help explain why some people with the allergy experience relatively mild reactions, while others develop severe, or even fatal, symptoms. “There’s a big question around why some patients are more susceptible to more severe reactions,” says Rodrigo Jiménez-Saiz at the Autonomous University of Madrid in Spain. Peanut allergy occurs when the immune system mistakenly identifies proteins in the legume as a threat, causing it to produce excessive amounts of a particular type of antibody. This ramps up inflammation, leading to symptoms such as itching, swelling and vomiting. In extreme cases, peanut exposure causes anaphylaxis, a life-threatening reaction that typically involves breathing difficulties. Jiménez-Saiz and his colleagues wondered whether the microbes that live on and within us play a role here, given the huge influence that our body’s various microbiomes have on our immune system. To find out, they inserted a small amount of peanut into the stomachs of three groups of mice without any …