Not all allergies are created equal, and oral allergy syndrome (OAS) is one of the lesser-known kinds of conditions. It causes your body to interpret certain fruits or vegetables as pollen.
Celery is one of the common culprits that sets off OAS (especially in areas of central Europe). It can make your skin irritated and itchy and causes hives or swelling around your mouth. Other reactions include nausea, asthma and even anaphylactic shock [source: Lawley].
You can cook some fruits and vegetables to avoid these kinds of reactions. But celery can trigger OAS no matter how you serve it. This means that when celery is used in spices and hidden in processed food, such as soups and casseroles, it can still affect someone who's allergic to it.