Mouth itching and swelling related to raw fruits
Mouth and throat itching and mild swelling immediately after eating fresh fruits like apples, peaches, pears, cherries, melons, kiwi, banana, and some vegetables like cucumber, zucchini, carrots and hazelnuts and almonds and other products like sunflower seeds, chamomile tea, and echinacea, can be experienced by people with ragweed and birch tree allergies. This is a result of the cross-reaction between the allergy antibodies that react to the pollen proteins with similar proteins found in these raw fruits and vegetables. Cooking and processing the food, usually the fruits and vegetables, may eliminate the reaction. Certain foods like hazelnuts may still give problems even if they are cooked.
Oral allergy syndrome usually causes mild symptoms which are relieved by an antihistamine. Most people simply avoid eating the raw fruit.
Source: Allergy Asthma Proc. 2005 May-Jun;26(3):229-30.