I can't eat apples. I also can't eat peaches, plums, pears, cherries, carrots, or celery. If I do, my mouth immediately feels like it's on fire, and my palate itches so badly I kick myself—again. I, like millions of people, have a condition called Oral Allergy Syndrome, which leaves some of the tastiest and most nutrient-rich vegetables and fruits just out of reach. Allergists estimate that 70 percent of people with allergic rhinitis, or hay fever, have some form of Oral Allergy Syndrome. But many, and even their doctors, haven't heard of it.

Fruit-pollen syndrome, as it's also called, is far less dangerous than food allergies to milk, eggs, wheat products, or peanuts. These so-called priority allergies trigger systemic reactions and are more likely to induce a fatal response, says Antony Ham Pong, a consultant in allergy and asthma at Children's Hospital of Eastern Ontario in Ottawa. My condition has more in common with localized contact allergies to latex or dyes in beauty products, for example, which cause skin hives and redness.

It's that pesky allergy to tree, grass, and weed pollens that explains why a carrot stick or a wedge of watermelon can become the enemy. The pollen molecules of certain trees (birch, alder, and poplar, for example) and weeds are so similar in structure to the proteins in certain raw fruits and vegetables that one of the body's natural defenses against allergens—an antibody called immunoglobulin E—can't tell the difference. When the antibody binds to the fruit or vegetable protein, anyone allergic to the similar pollens may develop an immune response that usually lasts only for a few, if uncomfortable, minutes. In rare instances, the reaction can bypass the mouth and enter the gastrointestinal tract, causing cramps and nausea.

If you're allergic to birch pollen, say, you might be prone to an oral allergy to apples, pears, peaches, plums, apricots, cherries, hazelnuts, and kiwis. "Ragweed pollen," says Warren Carr, an allergist in Mission Viejo, Calif., who served on the board of regents of the American College of Allergy, Asthma & Immunology, "has a high degree of cross-reactivity with the gourd family—watermelon, cantaloupe, zucchini, and cucumber." An allergy to the weed mugwort can spell trouble with vegetables and spices of the parsley family, including carrots, celery, dill, and cumin. Cruciferous vegetables like broccoli and cauliflower, lettuce, green onions, and cabbage as well as citrus, tropical, and berry fruits typically do not cause oral allergy reactions.

When too much of a problem fruit or vegetable is ingested at one time, severe responses can occur; anaphylaxis affects between 1 and 2 percent of OAS sufferers, notes William Anderson, an allergist from Bellingham, Wash., and a spokesperson for the American Academy of Allergy, Asthma and Immunology. These individuals will want to carry an epinephrine shot with them at all times.