Shellfish allergy symptoms generally develop within minutes to an hour of eating shellfish. They may include:

• Hives, itching or eczema (atopic dermatitis)

• Swelling of the lips, face, tongue and throat, or other parts of the body

• Wheezing, nasal congestion or trouble breathing

• Abdominal pain, diarrhea, nausea or vomiting

• Dizziness, lightheadedness or fainting

Allergies can cause a severe, potentially life-threatening reaction known as anaphylaxis. An anaphylactic reaction to shellfish or anything else is a medical emergency that requires treatment with an epinephrine (adrenaline) injection and a trip to the emergency room.

Signs and symptoms of anaphylaxis include:

• A swollen throat or a lump in your throat (airway constriction) that makes it difficult for you to breathe

• Shock, with a severe drop in your blood pressure

• Dizziness, lightheadedness or loss of consciousness

All food allergies are caused by an immune system overreaction. In shellfish allergy, your immune system mistakenly identifies a certain protein in shellfish as harmful, triggering the production of antibodies to the shellfish protein (allergen). The next time you come in contact with the allergen, your immune system releases histamine and other chemicals that cause allergy symptoms.

Types of shellfish

There are several types of shellfish, each containing different proteins:

• Crustaceans include crabs, lobster, crayfish, shrimp and prawn.

• Mollusks include squid, snails, clams, oysters and scallops.

Some people are allergic to only one type of shellfish but can eat others. Other people with shellfish allergy must avoid all shellfish.

You're at increased risk of developing a shellfish allergy if allergies of any type are common in your family.

Though people of any age can develop a shellfish allergy, it's more common in adults. Among adults, shellfish allergy is more common in women. Among children, shellfish allergy is more common in boys.

In severe cases, shellfish allergy can lead to anaphylaxis, a dangerous allergic reaction marked by a swollen throat (airway constriction), rapid pulse, shock, and dizziness or lightheadedness. Anaphylaxis can be life-threatening.

When you have shellfish allergy, you may be at increased risk of anaphylaxis if:

• You have asthma

• You have allergic reactions to very small amounts of shellfish (extreme sensitivity)

• You have a history of food-induced anaphylaxis

Anaphylaxis can be treated with an emergency injection of epinephrine (adrenaline). If you are at risk of having a severe allergic reaction to shellfish, you always should carry injectable epinephrine (EpiPen, Adrenaclick, others).