Shellfish allergy

Shellfish allergy is one of the most common food allergies. If you have a shellfish allergy, you may have an allergic reaction to only certain kinds of shellfish, or you may have an allergy to all shellfish.

Garvan J. Lynch
MBA (Public Health)

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What is it?

Shellfish allergy is one of the most common food allergies. If you have a shellfish allergy, you may have an allergic reaction to only certain kinds of shellfish, or you may have an allergy to all shellfish. Shellfish include marine animals with shells, such as clams, lobster and shrimp, as well as octopus and squid.

Shellfish allergy can cause mild symptoms, such as hives or nasal congestion, or more-severe and even life-threatening symptoms. For some people, even a tiny amount of shellfish can cause a serious reaction.

If you think you have a shellfish allergy, talk to your doctor. Tests can help confirm a shellfish allergy, so you can take steps to avoid future reactions.


Shellfish allergy symptoms include:

  • Hives, itching or eczema
  • Swelling of the lips, face, tongue and throat, or other parts of the body
  • Wheezing, nasal congestion or trouble breathing
  • Abdominal pain, diarrhoea, nausea or vomiting
  • Dizziness, lightheadedness or fainting
  • Tingling in the mouth

A severe allergic reaction to shellfish called anaphylaxis is rare — but can be life-threatening if it interferes with your breathing. An anaphylactic reaction 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
  • Rapid pulse
  • Dizziness, lightheadedness or loss of consciousness

Though they share similar symptoms, a shellfish allergy is different from an adverse reaction to toxins or bacteria in your food. Unlike an allergy, this common type of food poisoning doesn't directly involve your immune system and occurs only when you eat food that has been contaminated. An allergic reaction to shellfish usually occurs every time you eat the type of shellfish that causes the reaction.


All food allergies are caused by an immune system malfunction. Your immune system identifies certain shellfish proteins as harmful, triggering the production of antibodies to neutralize the shellfish protein (allergen). The next time you come in contact with proteins in shellfish, these antibodies recognize them and signal your immune system to release histamine and other chemicals that cause allergy symptoms.

Histamine and other body chemicals cause a range of allergic signs and symptoms. Histamine is partly responsible for most allergic responses, including runny nose, itchy eyes, dry throat, rashes and hives, nausea, diarrhoea, difficulty breathing, and in some cases, anaphylactic shock.

There are several types of shellfish, and each kind contains different allergy-causing proteins.

Crustaceans include crabs, lobster, crayfish, shrimp and prawns.

Mollusks include:

  • Bivalves, such as clams, mussels, oysters and scallops
  • Gastropods, such as limpets, periwinkles, snails (escargot) and abalone
  • Cephalopods, such as squid, cuttlefish and octopuses

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

Risk factors

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 most 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 should carry injectable epinephrine (such as an EpiPen) with you at all times.


Your doctor will ask about your symptoms and may perform a physical exam to find or rule out other medical problems. He or she may also recommend one or both of the following tests:

  • Skin test. In this test, your skin is pricked and exposed to small amounts of the proteins found in shellfish. If you're allergic, you'll develop a raised bump (hive) at the test location on your skin. This test is usually completed and interpreted by an allergy specialist.
  • Blood test. Also called allergen-specific IgE antibody test, radioallergosorbent (RAST) test or allergy screen, this test can measure your immune system's response to shellfish proteins by measuring the amount of certain antibodies in your bloodstream, known as immunoglobulin E (IgE) antibodies. A blood sample is sent to a medical laboratory, where it can be tested for evidence of sensitivity to shellfish proteins.

A history of allergic reactions shortly after exposure to shellfish can be a sign of a shellfish allergy, but allergy testing is the only sure way to tell what's causing your symptoms. Adverse reactions to shellfish are also sometimes caused by a nonallergic reaction, such as food poisoning or a bacterial or viral infection.


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