Oral Allergy Syndrome, also known as OAS, is caused by a cross-reaction between plant proteins from pollen, and fruits or vegetables. When an individual with a pollen allergy eats a fresh fruit or vegetable, the immune system sees a similarity between the pollen and the food--which causes an allergic reaction.
Most individuals with oral allergy syndrome can not eat the foods when raw but can eat them when cooked. It is suspected that cooking the food alters the protein enough so that the immune system does not recognize the food protein as being the same as the pollen.
Some symptoms of OAS include itching or swelling of the lips, mouth and throat that normally appear within minutes of eating the food item. Those most affected typically have asthma and/or pollen allergies and eat raw fruit.
If you think you have OAS, the best thing to do is avoid the food that causes reactions. If reactions are more severe than the symptoms listed above, one should see an allergist. It is possible that the reaction is not Oral Allergy Syndrome but a more severe food allergy.
List of Potential Pollen/Food Cross Reactions:
Ragweed: Bananas, melons (watermelon, cantaloupe, honeydew) zucchini, cucumber, dandelions, chamomile tea
Birch: Apples, pears, peaches, apricots, cherries, plums, nectarines, prunes, kiwi, carrots, celery, potatoes, peppers, fennel, parsley, coriander, parsnips, hazelnuts, almonds, walnuts
Grass: Peaches, celery, melons, tomatoes, oranges
Mugwort: Celery, apple, kiwi, peanut, fennel, carrots, parsley, coriander, sunflower, peppers
Alder: Celery, pears, apples, almonds, cherries, hazelnuts, peaches, parsley
Latex: Bananas, avocado, kiwi, chestnut, papaya