What Is a Shellfish Allergy?

A shellfish allergy is not exactly the same as a seafood allergy. Seafood includes fish (like tuna or cod) and shellfish (like lobster or clams). Even though they both fall into the category of "seafood," fish and shellfish are biologically different. So fish will not cause an allergic reaction in someone with a shellfish allergy, unless that person also has a fish allergy.

Shellfish fall into two different groups:

• crustaceans, like shrimp, crab, or lobster
• mollusks, like clams, mussels, oysters, scallops, octopus, or squid

Some people with shellfish allergies are allergic to both groups, but some are allergic only to one.

Most allergic reactions to shellfish happen when someone eats shellfish. But sometimes a person can react to touching shellfish or breathing in vapors from cooking shellfish.

Shellfish allergy can develop at any age. Even people who have eaten shellfish in the past can develop an allergy. Some people outgrow certain food allergies over time, but those with shellfish allergies usually have the allergy for the rest of their lives.

What Are the Signs & Symptoms of a Shellfish Allergy?

When someone is allergic to shellfish, the body's immune system, which normally fights infections, overreacts to proteins in the shellfish. Every time the person eats (or, in some cases, handles or breathes in) shellfish, the body thinks these proteins are harmful invaders and releases chemicals like . This can cause symptoms such as:

• wheezing
• trouble breathing
• coughing
• hoarseness
• throat tightness
• belly pain
• vomiting
• diarrhea
• itchy, watery, or swollen eyes
• hives
• red spots
• swelling
• a drop in blood pressure, causing lightheadedness or loss of consciousness (passing out)

Allergic reactions to shellfish can differ. Sometimes the same person can react differently at different times. Some reactions can be very mild and involve only one system of the body, like hives on the skin. Other reactions can be more severe and involve more than one part of the body.

Shellfish allergy can cause a severe reaction called anaphylaxis, even if a previous reaction was mild. Anaphylaxis might start with some of the same symptoms as a less severe reaction, but can quickly get worse. The person may have trouble breathing or pass out. More than one part of the body might be involved. If it isn't treated, anaphylaxis can be life-threatening.

How Is an Allergic Reaction to Shellfish Treated?

If you have a shellfish allergy (or any kind of serious food allergy), the doctor will want you to carry an epinephrine auto-injector in case of an emergency.

An epinephrine auto-injector is a prescription medicine that comes in a small, easy-to-carry container. It's easy to use. Your doctor will show you how. Keep it nearby, not in a locker or in the nurse's office.

Sometimes allergists recommend also carrying over-the-counter (OTC) , as these can help treat mild allergy symptoms. Use an antihistamine after — not as a replacement for — the epinephrine shot during a life-threatening reaction.

The doctor can also give you an allergy action plan, which helps you prepare for, recognize, and treat an allergic reaction. Share the plan with anyone else who needs to know, such as relatives, school officials, and coaches. Also consider wearing a medical alert bracelet.

Every second counts in an allergic reaction. If you start having serious allergic symptoms, like swelling of the mouth or throat or trouble breathing, use the epinephrine auto-injector right away. Also use it right away if your symptoms involve two different parts of the body, like hives with vomiting. Then call 911 and have someone take you to the emergency room. You need to be under medical supervision because even if the worst seems to have passed, a second wave of serious symptoms can happen.

What Else Should I Know?

If you have a shellfish allergy, you must not eat shellfish. You also must not eat any foods that might contain shellfish as ingredients. Anyone who is sensitive to the smell of cooking shellfish should avoid restaurants and other areas where shellfish is being cooked.

Always read food labels to see if a food contains shellfish. Manufacturers of foods sold in the United States must state whether foods contain any of the most common allergens, including crustacean shellfish. The label should list "shellfish" in the ingredient list or say "Contains shellfish" after the list.

Some foods look OK from the ingredient list, but while being made they can come in contact with fish. This is called cross-contamination. Look for advisory statements such as "May contain shellfish," "Processed in a facility that also processes shellfish," or "Manufactured on equipment also used for shellfish." Not all companies label for cross-contamination, so if in doubt, call or email the company to be sure.

Manufacturers also do not have to list mollusk shellfish ingredients because mollusk shellfish (clams, mussels, oysters, or scallops) are not considered a major food allergen. When labels say a food contains shellfish, they refer to crustacean shellfish. Contact the company to see about cross-contamination risk with mollusks.

Cross-contamination often happens in restaurants. In kitchens, shellfish can get into a food product because the staff use the same surfaces, utensils (like knives, cutting boards, or pans), or oil to prepare both shellfish and other foods.

This is particularly common in seafood restaurants, so some people find it safer to avoid these restaurants. Shellfish is also used in a lot of Asian cooking, so there's a risk of cross-contamination in Chinese, Vietnamese, Thai, or Japanese restaurants. When eating at restaurants, it may be best to avoid fried foods because many places cook chicken, French fries, and shellfish in the same oil.

When eating away from home, make sure you have an epinephrine auto-injector with you and that it hasn't expired. Also, tell the people preparing or serving your food about the shellfish allergy. Sometimes, you may want to bring food with you that you know is safe. Don't eat at the restaurant if the chef, manager, or owner seems uncomfortable with your request for a safe meal.

Also talk to the staff at school about cross-contamination risks for foods in the cafeteria. It may be best to pack lunches at home so you can control what's in them.

For more about managing food allergies, visit:

• Food Allergy Research and Education Network (FARE).