When you bite into a fresh apple and your mouth starts to tingle, cross-reactivity may be why. Proteins in birch pollen share a similar shape with proteins in apples, so the immune system mistakes one for the other — triggering tingling and swelling where the raw food touches the lips and mouth. This is oral allergy syndrome, and it mainly affects people who are also sensitive to tree or grass pollens. To study this shape-based confusion, researchers used two model systems to measure how strongly foods trigger allergic responses and how much they cross-react with each other.