Oral Allergy Syndrome, also known as pollen-food syndrome is caused by cross-reactive allergens found in both pollen and raw fruits, vegetables, and some tree nuts. Those with oral allergy syndrome typically have issues with the inhalants: birch, ragweed, and grass.  The cross reactors are as follows:

Birch Pollen: Apple, almond, carrot, celery, cherry, hazelnut, kiwi, peach, pear, plum.

Grass Pollen: celery, melons, oranges, peaches, tomato.

Ragweed Pollen: Bananna, cucumber, melons, sunflower seeds, Zucchini.