Oral allergy syndrome can progress to a severe allergic reaction, but this is very uncommon. This type of reaction occurs in the heat and acid-labile proteins are found in certain fresh fruits and vegetables. These delicate proteins are denatured (broken down) by heat and acid, and therefore if cooked or processed, patients will not react. This also explains why the reaction does not progress beyond oral symptoms because as soon as the food reaches the acid in the stomach, it again is denatured (or broken down). The proteins that lead to oral allergy syndrome are known to cross-react with certain pollen.