What causes the development of an oral allergy syndrome?

OAS: This is a disorder related to cross reactive proteins in the fresh or raw fruit of vegetables you eat. For example, fresh apple causes itch, tingle, and possible swelling in the mouth, but apple pie, apple sauce, apple crumble cause no symptoms. It is because you have seasonal pollen allergies. Think where the fruit or veggie grows and it can give you a clue to what pollens you are allergic to.