Oral allergy syndrome, also called pollen-food syndrome, happens because proteins in some raw fruits, vegetables, and nuts are similar in shape to pollen proteins. In people who are allergic to pollen, the immune system can cross-react, causing an itchy or tingling mouth, lips, or throat, and sometimes mild lip swelling, shortly after eating the food. Common triggers include apples, peaches, cherries, kiwi, celery, carrots, and some nuts, and symptoms are often worse during the relevant pollen season.
The reaction is usually mild and confined to the mouth because the proteins are easily broken down by heat and digestion. For this reason, the same foods are often tolerated when cooked, baked, or tinned, even if the raw version causes symptoms.
Although most reactions are mild, a small number of people can have more significant reactions, particularly to certain foods, so it is worth getting advice rather than assuming all reactions will stay mild. See a doctor if your symptoms spread beyond the mouth, involve swelling or breathing difficulty, or if you are unsure which foods are safe, and seek emergency care for any reaction affecting breathing.