Raven, a 28-year old long distance runner, had a lengthy history of increased sneezing with a runny nose in the fall. After undergoing skin tests, her doctor diagnosed a ragweed allergy. She followed her doctor’s advice and began her daily run in the early morning. Her condition seemed under control. However, one day, when she arrived home, her thoughtful husband, Tony, gave her several pieces of watermelon. Minutes after eating the fruit, she developed itchiness and swelling of the mouth and tongue, as well as tightness of her throat. Raven yelled at him, “Are you trying to poison me?”

Oral Allergic Syndrome (OAS), also called pollen-food allergyis an allergic reaction to fruits (usually fresh), nuts, and vegetables. OAS typically develops in adult hay fever sufferers. In adults, up to 60% of all food allergic reactions are due to cross-reactions between foods and inhalant allergens. Well-cooked, canned, pasteurized or frozen food usually causes little or no reaction because of the breakdown of cross-reacting proteins during processing.

Raven inherited the ability to produce IgE antibodies to ragweed that bind themselves to the surface mast cells and basophil cells in the lining of her eyes, nose, airways and even oral cavity. After exposure to ragweed pollen, the mast cells and basophil cells immediately release a variety of potent chemical mediators which induce allergic symptoms: itchy and watery eyes, runny and stuffy nose, and even wheezing and difficulty breathing.

Raven developed OAS due to her allergic reaction to watermelon protein, not ragweed pollen. The protein of ragweed pollen is structurally similar to watermelon. Therefore, IgE antibodies can bind both pollen proteins and structurally similar food proteins (cross reaction). Consequently, she developed allergy symptoms in two different situations: hay fever (in the presence of ragweed pollen) and food allergy (in the presence of watermelon).

Many OAS sufferers develop OAS within 5 minutes of contact with the offending food, and almost all present symptoms within 30 minutes. People allergic to ragweed frequently experience associated OAS reactions to banana, watermelon, cantaloupe, honeydew, zucchini and cucumber.  This does not mean that she will experience adverse effects from all of these foods. However, reactions may begin with one type of food and develop to others later.

Aside from ragweed allergy, OAS is also common in people with tree or grass allergies. Foods that can trigger a reaction in people with these allergies include peaches, apricots, celery, oranges, potatoes, tomatoes, melons, apples, pears, cherries, carrots, hazelnuts, kiwis, and almonds.

Finally, the verdict is in—Raven’s husband is innocent! She developed oral allergy syndrome to watermelon in conjunction with her ragweed allergy. She should avoid watermelon and ragweed-related fruits during ragweed season.  And, hopefully, she can enjoy some of her favorite fruits when ragweed season is over.