In several studies, a cross-reactivity has been reported between birch pollen and nuts. This cross¬ reactivity shows itself in a syndrome that is known as the para-birch syndrome. The complaints of people suffering from this syndrome result from a birch pollen allergy (sneezing, nasal obstruction, and conjunctivitis during the birch pollen season) and also from an allergy to nuts and/or certain fruits. Allergic reactions to these foods mainly cause symptoms such as itching in and around the mouth and pharynx and swelling of the lips. In some cases, however, more severe reactions occur. Related fruits in this context are apple, peach, plum, cherry, and orange. Also, some vegetables such as celery and carrot have been shown to be cross-reactive with the birch allergen. Other known cross-reactivity combinations are grass pollen with carrot, potato, wheat, and celery. A grass pollen-allergic person may become allergic to wheat as well. The exact mechanisms underlying these phenomena are not known.