When you have a shellfish allergy and you eat shellfish, your body reacts as if the shellfish are trying to cause harm. It fights back by setting off an allergic reaction. A mild reaction may include a few raised, red, itchy patches of skin (called hives). A severe reaction may cause hives all over, swelling in the throat, trouble breathing, nausea or vomiting, or fainting. This is called anaphylaxis (say "ANN-uh-fuh-LAK-suss"). It can be deadly.
A good way to prevent an allergic reaction is to avoid the foods that cause it. Shellfish might be used as ingredients in surimi (imitation shellfish, which is often found in sushi) and fish sauce. An allergy doctor or a dietitian may be able to help you understand which foods might be okay and what to avoid. Learn what to do if you have a reaction.