Oral allergy syndrome is also known as Pollen Food Allergy syndrome. It is caused by cross reactivity of pollen with raw fruits and vegetables. The body is unable to differentiate between what is pollen and what is a fruit, so symptoms like itching in the mouth, swelling of the lips, and discomfort in the throat may be experienced. If the fruit is cooked, the heat changes the shape of the protein, so the body no longer confuses the protein with pollen. For example, a raw apple may cause tingling in the mouth if a person is allergic to tree pollen; however, apple pie or apple sauce can be enjoyed without any discomfort because the apples are cooked. It is estimated that 1 in 3 people with seasonal allergies also experience oral allergy syndrome.
Treatment for this condition, besides avoidance of the fruits and vegetables that trigger these symptoms, is allergy immunotherapy or allergy shots. These shots are a treatment course that change the way the body responds to pollen. If the body is less responsive to pollen then it will also be less responsive to the fruits and vegetables that cross react to the pollen.
Please check out this great chart regarding Oral Allergy Syndrome from the Allergy & Asthma Network.