Allergic reactions to fruits and vegetables are usually mild and often they just affect the mouth, causing itching or a rash where the food touches the lips and mouth. This is called oral allergy syndrome. A number of people who react in this way to fruit or vegetables will also react to tree and weed pollens. So, for example, people who are allergic to birch pollen are also likely to be allergic to apples.
Cooking can destroy a number of the allergens in fruits and vegetables, so cooked fruit often won't cause a reaction in people with an allergy to fruit. Pasteurised fruit juice might not cause an allergic reaction either, for the same reason. However, the allergens in some vegetables, such as celery, aren't affected by cooking. Some fruits and vegetables, such as tomatoes, are more likely to cause a reaction as they get riper.
Howard M. Druce MD, FACP is a board-certified physician in allergy/immunology who is in private practice in Somerville, New Jersey. My practice is for both children and adults. I am Clinical Professor of Medicine at UMDNJ-New Jersey Medical School in Newark, NJ and a fellow of both the American Academy and College of Allergy, Asthma and Immunology. I am an active member of scientific committees of both professional societies.