DEAR DOCTOR K: Last month I broke out in hives after eating oysters. I had a blood test, which came back negative for a shellfish allergy. Why does my doctor still want me to do a food challenge?
DEAR READER: Allergic reactions occur when your body's immune system overreacts against a harmless substance -- in your case, possibly, shellfish. Food allergies can cause a variety of symptoms that range from mild to life-threatening.