Shellfish vs. Mollusc: Understanding Your Seafood Diagnostic Profile

Shellfish allergy is one of the most common food allergies in UK adults, yet the term "shellfish" is widely misunderstood. Many people assume that if they are allergic to prawns, they must avoid all seafood — from mussels to squid to fish fingers. In reality, the picture is more nuanced. Crustaceans and molluscs are biologically distinct groups with different proteins, different patterns of food allergy sensitisation, and different implications for your diet. This article explains why shellfish allergy symptoms can vary dramatically depending on which species you react to — and how targeted IgE blood testing can help you and your clinician build a clearer diagnostic picture.

Shellfish, Crustaceans, Molluscs: What These Terms Really Mean

The word "shellfish" is a culinary term, not a strict biological category. In everyday language, it covers a wide range of marine and freshwater species. For allergy purposes, it is essential to distinguish between two main groups:

Crucially, finned fish — such as cod, salmon, tuna, and mackerel — are not shellfish. Fish allergy involves entirely different proteins (primarily parvalbumins) and is classified separately under UK food labelling regulations. Being allergic to shellfish does not predict fish allergy, and vice versa (NHS, 2024).

Under the Food Standards Agency's allergen framework, crustaceans and molluscs are listed as two of the 14 named allergens that must be declared on food labels and menus in the UK. They are separate categories — an important distinction for anyone with a confirmed allergy to one group but not the other.

Why Reactions Can Cluster: Shared Proteins Like Tropomyosin

The primary allergen driving crustacean allergy in most people is a muscle protein called tropomyosin. This protein is highly conserved across species — meaning it has a very similar molecular structure in prawns, crab, lobster, and even shrimp. This explains why prawn allergy often co-occurs with crab or lobster allergy: the IgE antibodies recognise the same protein across multiple crustacean species.

But tropomyosin is not unique to shellfish. It is also found in:

• House dust mites — the most clinically significant cross-reactor. People with dust mite allergy may produce IgE against dust mite tropomyosin, which can cross-react with crustacean tropomyosin on blood tests. This can result in a positive crustacean IgE result even in someone who eats shellfish without symptoms.
• Cockroaches — tropomyosin-based cross-reactivity has been documented between cockroach and crustacean allergens (BSACI, 2024).
• Molluscs — some mollusc species also contain tropomyosin, though the degree of cross-reactivity with crustacean tropomyosin varies. Squid tropomyosin, for example, is structurally more distant from prawn tropomyosin than crab tropomyosin is.

This is why a blanket "shellfish allergy" label can be misleading. You may be sensitised to crustacean tropomyosin but tolerate bivalve molluscs perfectly well — or your positive test may reflect dust mite cross-reactivity rather than a genuine seafood allergy. Understanding which proteins are driving your results is key to building an accurate diagnostic profile. Our guide to understanding your allergy lab report explains what IgE values and component results mean and why professional clinical interpretation is essential.

Symptoms and Timing: Allergy vs Food Poisoning vs Intolerance

Seafood reactions are not always allergic. Before considering testing, it is helpful to understand the three most common types of adverse reaction to shellfish:

If you are unsure whether your reaction was allergic, toxic, or coincidental, a food and symptom diary can help identify patterns before any blood testing is arranged.

What to Test: Specific IgE Panels and Component Clues

When investigating a potential shellfish allergy, your clinician may request specific IgE blood tests for one or more of the following:

Component testing is particularly useful in shellfish allergy because tropomyosin is the major cross-reactive protein. If a patient tests positive to whole shrimp extract but negative to shrimp tropomyosin (rPen a 1), the positive result may be driven by other shrimp proteins rather than the tropomyosin that cross-reacts with dust mites. Conversely, a positive tropomyosin result in someone with a known dust mite allergy may represent cross-reactivity rather than genuine seafood sensitisation (BSACI, 2024).

These distinctions are why targeted testing — guided by the patient's clinical history — is more informative than broad "seafood panel" testing without clinical context. You can browse our available allergy tests to see which specific allergens and components we offer.

Understanding a Mixed Report: Positive to Some, Negative to Others

It is very common to receive a report showing positive IgE for some shellfish species and negative for others. For example, you might be positive for prawn and crab (both crustaceans) but negative for mussel and squid (both molluscs). This is not unusual and does not mean the test was unreliable.

A mixed result pattern often reflects genuine immunological differences between the two groups:

• Crustacean-only sensitisation — the most common pattern. IgE directed against crustacean tropomyosin may not cross-react significantly with mollusc proteins.
• Mollusc-only sensitisation — less common but well documented, particularly for cephalopods (squid, octopus) or bivalves.
• Dual sensitisation — positive results to both groups. This may reflect shared tropomyosin or independent sensitisation to different proteins.
• Cross-reactive positivity with dust mites — tropomyosin-driven IgE from dust mite sensitisation can produce false positives across both groups, particularly at low IgE levels.

Regardless of the pattern, the clinical principle is the same: do not reintroduce any food you have been avoiding based on test results alone. A negative IgE result makes allergy less likely but does not exclude it with certainty. Reintroduction of avoided foods — particularly after a previous severe reaction — should only occur under clinical supervision, typically through a supervised oral food challenge arranged by your allergy specialist (NICE, 2024).

If you are in the process of exploring your results, our allergy testing London service can provide you with a detailed laboratory report to share with your GP or specialist for clinical interpretation.

Eating Out, Travel, and UK Labelling Realities

For anyone managing a confirmed shellfish or mollusc allergy, navigating restaurants and food labels is a daily reality. Here is what UK regulations require — and what they do not cover:

Pre-Packed Food

Under UK food labelling law (enforced by the Food Standards Agency), crustaceans and molluscs must be clearly declared in the ingredients list of all pre-packed food. They are listed as two separate allergen categories among the 14 named allergens. If a product contains prawn but not mussel, the label should state this — but it does not distinguish between crustacean species (e.g., "crustaceans" may be used as a generic term).

Restaurants, Cafés, and Takeaway

Under Natasha's Law and PPDS labelling rules, allergen information must be available for all food sold, including prepacked for direct sale items. However:

• Cross-contamination is not always declared. "May contain" warnings are voluntary, not legally required. In a restaurant kitchen that handles multiple types of seafood, cross-contact between crustaceans and molluscs is common.
• Shared cooking surfaces. Deep-fryers, grills, and preparation surfaces in fish restaurants or chip shops may be used for both crustaceans and fish — a particular risk for people allergic to one but not the other.
• Asian and Mediterranean cuisines frequently use shellfish pastes, fish sauce, or dried shrimp as flavour bases. These ingredients may not be obvious on the menu.

Travelling Abroad

Allergen labelling rules vary significantly between countries. When travelling, carry a translated allergy card listing the specific species you need to avoid (e.g., "prawns, crab, lobster" rather than "shellfish"). Anaphylaxis UK provides free downloadable allergy translation cards in multiple languages.

Diagnostic-Only Pathway at Our Clinic

At AllergyClinic.co.uk, we offer a nurse-led venepuncture service that collects your blood sample for laboratory analysis. We do not provide doctor consultations, clinical interpretation, diagnosis, or treatment recommendations as part of this service. Our role is diagnostic sampling — providing you with a clear, detailed IgE report that you can take to your GP or allergy specialist.

For shellfish and mollusc investigation, we offer individual allergen tests (prawn, crab, mussel, squid, and others) as well as component tests such as tropomyosin where relevant. Your report will include specific IgE values expressed in kU/L alongside reference ranges, allowing your clinician to interpret the findings in the context of your symptoms and dietary history.

If you have experienced reactions to seafood and want objective data before your next clinical appointment, our testing service provides a straightforward first step.

Questions to Ask After You Receive Your Report

Once you have your IgE results, these questions can guide a productive conversation with your GP or allergy specialist:

1. Am I sensitised to crustaceans, molluscs, or both?
2. Could my positive result be explained by dust mite cross-reactivity?
3. Is component testing (e.g., tropomyosin) needed to clarify the picture?
4. Which specific species do I need to avoid, and which may be safe?
5. Should I be referred for a supervised oral food challenge?
6. Do I need to carry an adrenaline auto-injector?
7. How should I manage cross-contamination risk when eating out?

Frequently Asked Questions

Glossary

IgE (Immunoglobulin E)

A type of antibody produced by the immune system in response to an allergen. Elevated specific IgE levels indicate sensitisation.

kU/L (kilounits per litre)

The unit of measurement for specific IgE levels in blood tests. Higher values indicate greater sensitisation, but do not directly predict reaction severity.

Component testing

A refined form of IgE testing that measures antibodies against individual proteins (components) within an allergen, rather than the whole extract.

Tropomyosin

A muscle protein found in crustaceans, molluscs, dust mites, and insects. It is the primary allergen responsible for crustacean allergy and the main cause of cross-reactivity between shellfish and dust mites.

Cross-reactivity

When IgE antibodies produced against one allergen also recognise a structurally similar protein in a different species, potentially causing a positive test result without clinical allergy.

Sensitisation

The presence of specific IgE antibodies to an allergen. Sensitisation does not automatically equal clinical allergy — it means the immune system has been exposed and has produced antibodies.

Oral food challenge

A supervised clinical procedure in which a patient eats gradually increasing amounts of a food under medical observation to determine whether they are truly allergic. This is the gold standard for confirming or excluding food allergy.

Sources

• NHS — Food allergy overview and anaphylaxis guidance (2024): nhs.uk/conditions/food-allergy
• Allergy UK — Shellfish allergy factsheet (2024): allergyuk.org
• Anaphylaxis UK — Emergency management and allergy action plans (2024): anaphylaxis.org.uk
• BSACI — Guidelines on the investigation and management of food allergy, including component-resolved diagnostics (2024): bsaci.org
• Food Standards Agency — UK allergen labelling guidance for the 14 named allergens (2024): food.gov.uk
• NICE — Anaphylaxis: assessment and referral after emergency treatment (CG134) and food allergy in under 19s (CG116) (2024): nice.org.uk