Like other allergies, a tomato allergy is caused by a histamine reaction to a certain protein, in this case usually one found in the tomato's juice, seeds, or skin. Symptoms include swelling or itching near the mouth, hives, difficulty breathing, or a rash. Diarrhea and vomiting may also occur.
Interestingly, some individuals with a tomato allergy are able to eat cooked or processed tomatoes with no adverse reaction. However, tomato allergic individuals may have trouble with foods related to tomato plants like potatoes, tobacco, and eggplant.
As with any suspected allergy, it is necessary to avoid the food that causes a reaction. For a diagnosis confirmation, visit an allergist.