The proteins found in some fruits and vegetables are very similar to those found in particular pollen. These proteins can confuse the immune system in people with food or outdoor allergies. The result is called oral allergy syndrome (OAS). For instance, if you are allergic to birch tree pollen, a primary airborne allergen responsible for symptoms in the springtime, you may have reactions triggered by peach, apple, pear, kiwi, plum, coriander, fennel, parsley, celery, cherry and carrot. The most frequent reaction involves itchiness or swelling of the mouth, face, lip, tongue and throat. Symptoms usually appear immediately after eating raw fruits or vegetables, although the reaction can occur more than an hour later. Rarely, OAS can cause severe throat swelling or anaphylaxis in a person who is highly allergic.