Shellfish allergy
Allergy to crustaceans and molluscs; often severe and lifelong.
About shellfish allergy
Shellfish allergy covers reactions to two groups: crustaceans such as prawn, crab and lobster, and molluscs such as mussel, scallop and squid. It is one of the most common food allergies in adults, frequently begins in adolescence or adulthood and is usually lifelong. The reaction is IgE-mediated, with the immune system producing antibodies to muscle proteins, above all tropomyosin, which triggers a rapid release of histamine and other chemicals on exposure.
Symptoms typically start within minutes and can include an itchy mouth, hives, swelling, vomiting, wheeze and anaphylaxis with breathing difficulty or a fall in blood pressure. Because tropomyosin is shared across shellfish species, many people react to several types, though some are allergic only to crustaceans or only to molluscs. The same protein family underlies cross-reactivity with house dust mite in certain individuals, and even cooking vapours can trigger reactions in the highly sensitive.
Diagnosis uses skin-prick or specific-IgE blood tests, sometimes with component testing or a supervised challenge to define which shellfish are safe. Management centres on strict avoidance, careful checking of restaurant meals, sauces and stocks, and vigilance about cross-contamination. Those at risk of severe reactions carry an adrenaline auto-injector and an action plan. A severe reaction is a medical emergency: use adrenaline immediately and call 999.
Common triggers
- Prawns, crab and lobster\nMussels, clams and oysters\nSeafood stock and sauces
Symptoms
- Itching or tingling in the mouth
- Hives, itching or eczema
- Swelling of the lips, face, tongue or throat
- Stomach cramps, nausea, vomiting or diarrhoea
- Wheezing or trouble breathing
- In severe cases, anaphylaxis
Avoidance & management
- Read ingredient labels every time
- Ask about ingredients and cross-contamination when eating out
- Keep the food out of shared cooking areas
- Carry prescribed antihistamines or an adrenaline auto-injector
- Seek emergency help for any severe reaction
Sources & references
Allergy safety: what everyone should know
Mild-to-moderate reaction signs
- Itching, hives or a raised rash
- Swelling of lips, face or eyes
- Tingling mouth, sneezing, runny nose
- Stomach pain, nausea or vomiting
Anaphylaxis — call 999 (UK) or 112 (EU) immediately
- Difficult or noisy breathing, wheeze, persistent cough
- Swelling of the tongue or tightness in the throat
- Dizziness, collapse, pale and floppy (young children)
- Use an adrenaline auto-injector (e.g. EpiPen) at once if prescribed, then call an ambulance even if symptoms improve
Allergy or intolerance? A true allergy involves the immune system and can escalate quickly; an intolerance (e.g. lactose intolerance) is unpleasant but not life-threatening. The symptoms overlap, so self-diagnosis is unreliable.
Getting tested: home-testing kits based on IgG are not diagnostically valid. If you suspect an allergy, ask a GP for referral to an allergy clinic for skin-prick or specific-IgE testing and a supervised food challenge where appropriate.