Oral allergy syndrome, also known as pollen-food syndrome,is caused by cross-reacting allergens found in both pollen and raw fruits, vegetables, or some tree nuts. The immune system recognizes the pollen and similar proteins in the food and directs an allergic response to it. People affected by oral allergy syndrome can usually eat the same fruits or vegetables in cooked form because the proteins are distorted during the heating process, so that the immune system no longer recognizes the food.
Oral allergy syndrome typically does not appear in young children; the onset is more common in older children, teens, and young adults who have been eating the fruits or vegetables in question for years without any problems. Those with oral allergy syndrome typically have allergy to birch, ragweed, or grass pollens.