Allergic reactions to vegetables may be relatively mild and limited to the mouth and throat (itching of the lips, mouth, throat, and swelling of the lips, tongue, throat, and palate). This is called oral allergy syndrome. These mild reactions may occur in people allergic to pollen (e.g. birch pollen) when they eat certain vegetables.
Cooking destroys the pollen related vegetable allergens, and thus cooked vegetables are often safe for people with pollen related vegetable allergy.
Other reactions to vegetables may be more severe for example celery allergy occurring most frequently in Central Europe. The celery allergen is heat stable and therefore also found in soups and other heated products.