What is shellfish allergy, and why does it suddenly appear in adults?

Shellfish Allergy Explained: Symptoms, Hidden Sources & What You Need to Know

A comprehensive guide to understanding crustacean allergies — including why so many people develop them as adults, seemingly out of nowhere

What Is Shellfish Allergy?

If you’ve recently had a reaction to prawns, crab, or lobster, you’re probably asking yourself the same question that thousands of people ask every year: “How can I be allergic to something I’ve eaten my whole life?”

Here’s what’s happening inside your body. Shellfish allergy occurs when your immune system mistakenly identifies proteins in crustaceans as harmful. The main culprit is a protein called tropomyosin. When you eat prawns (or crab, lobster, scampi, or crayfish), your immune system releases antibodies and chemicals — including histamine — that cause allergic symptoms.

The baffling thing about shellfish allergy is that it doesn’t usually start in childhood. Unlike peanut or milk allergies that typically appear in early years, shellfish allergy most commonly develops in adulthood — with the average diagnosis age being 28 years old. You might have enjoyed prawn cocktails at every Christmas dinner for three decades, then one day, your body decides prawns are the enemy.

Under UK food safety law, crustaceans are classified as FSA Allergen #3 — one of the 14 major allergens that food businesses must declare. This is separate from fish (Allergen #5) and molluscs like oysters and mussels (Allergen #8). This distinction matters because being allergic to shellfish doesn’t mean you’re allergic to fish — the proteins are completely different.

Why Do Shellfish Allergies Develop in Adulthood?

This is the question that haunts most people who suddenly develop a shellfish allergy. You’re not imagining things — and you’re not going mad. Around 60% of all shellfish allergies first appear in adulthood, which makes it genuinely unusual among food allergies.

Scientists believe several factors contribute to adult-onset shellfish allergy:

Cumulative exposure. Each time you eat shellfish, your immune system takes note. For some people, it takes years — even decades — of exposure before the immune system tips over into treating the proteins as threats. Think of it like filling a cup: one day, it simply overflows.

The dust mite connection. Here’s something fascinating that most people don’t know: the protein in shellfish (tropomyosin) shares 81% similarity with house dust mite proteins. If you’ve spent years being sensitised to dust mites — perhaps through hay fever, eczema, or asthma — your immune system may have unknowingly primed itself to react to shellfish too. Studies have even found shellfish sensitisation in people who follow kosher diets and have never eaten shellfish.

Immune system changes. Your immune system isn’t static. Stress, illness, hormonal changes, and simply getting older can all shift how your body responds to foods you’ve previously tolerated without issues.

The hardest part? Once you develop a shellfish allergy as an adult, it’s unlikely to go away. Unlike childhood egg or milk allergies where many children eventually outgrow them, adult-onset shellfish allergy is typically lifelong.

Symptoms of Shellfish Allergy

Shellfish allergy symptoms can range from mildly uncomfortable to genuinely life-threatening. What makes this allergy particularly concerning is the speed — reactions typically begin within minutes, though they can appear up to two hours after eating.

Where Shellfish Hides: Sources You Might Not Expect

Avoiding obvious shellfish — the prawn cocktail, the crab sandwich, the lobster thermidor — is straightforward enough. But shellfish has a way of sneaking into dishes where you’d never expect it. This is where restaurants often get it wrong.

Asian Cuisine: The Hidden Danger Zone

Shrimp paste is the foundation of many Thai curry pastes. When you ask “does this contain prawns?” the answer might be “no” — because the paste isn’t prawns, it’s shrimp paste. Same allergen, different name. Fish sauce in Vietnamese and Thai cooking can also contain crustacean derivatives. And those prawn crackers that arrive automatically at Chinese restaurants? Staff often don’t think to mention them because they’re “just crackers.”

Other Hidden Sources

• Worcestershire sauce — some brands include anchovies and shellfish
• Paella stock cubes — often contain shellfish extract for flavour
• Bouillabaisse and fish stews — typically made with mixed seafood stock
• Dim sum and dumplings — many varieties contain prawn
• Caesar salad dressing — some recipes include shellfish-based ingredients

One Story That Says It All

When restaurants fail to take shellfish allergies seriously, the consequences can be devastating. Jasmine’s story shows exactly what can happen — and why getting proper legal help matters.

How Shellfish Allergy Is Diagnosed

If you suspect you have a shellfish allergy — perhaps because you’ve had a reaction, or because of a family history — getting a proper diagnosis is important. Self-diagnosis isn’t reliable, and unnecessarily avoiding foods can affect your quality of life.

Your GP can refer you to an NHS allergy clinic, where specialists use several methods to confirm or rule out shellfish allergy:

Skin prick test. A tiny amount of shellfish extract is placed on your forearm, and the skin is lightly pricked. If you’re allergic, you’ll develop a small raised bump (wheal) within 15-20 minutes. This test is quick, relatively painless, and provides immediate results.

Blood test (specific IgE). A blood sample is tested for antibodies specific to shellfish proteins. This can be helpful if skin testing isn’t suitable — for example, if you’re taking antihistamines that might interfere with results.

Oral food challenge. Considered the gold standard but only performed in hospital settings where emergency treatment is available. You’re given gradually increasing amounts of shellfish under medical supervision. This definitively confirms whether you’re allergic and how sensitive you are.

A confirmed diagnosis also helps establish your history if you later need to make a compensation claim after a restaurant or food business fails to protect you from your known allergen.

Living With Shellfish Allergy

Finding out you have a shellfish allergy — especially as an adult who’s enjoyed seafood for years — can feel like a significant loss. Your favourite restaurant suddenly feels off-limits. Holiday destinations with famous seafood become sources of anxiety rather than excitement. It’s a genuine adjustment, and it’s okay to feel frustrated about it.

But most people with shellfish allergies live full, normal lives. Here’s what helps:

Carry your adrenaline auto-injector everywhere. If you’ve been prescribed an EpiPen or similar device, it needs to be with you at all times — not in your car, not at home, but on your person. Check expiry dates regularly. Many people carry two, just in case.

Learn to read labels properly. In the UK, crustaceans must be highlighted in bold on ingredient lists. But don’t just scan — read carefully. And remember that “may contain” warnings mean real risk, not just legal box-ticking.

Get confident about speaking up in restaurants. This is the hard one. Many people feel embarrassed about declaring allergies, worried about being seen as “difficult.” But your health matters more than mild awkwardness. A good restaurant will take your allergy seriously. A restaurant that doesn’t isn’t worth your money — or your safety.

Tell friends and family. Make sure the people close to you know about your allergy, recognise the signs of a reaction, and know where you keep your auto-injector. This matters especially for new partners, work colleagues, and anyone who might cook for you.

What To Do If You’ve Had a Reaction at a Restaurant or Takeaway

You did everything right. You told them about your allergy. You asked the right questions. And still, you had a reaction.

This happens more often than it should. Sometimes it’s a kitchen mistake. Sometimes it’s a language barrier. Sometimes — and this is the hardest one to accept — the restaurant simply didn’t take your allergy seriously enough.

Under UK law, food businesses have clear legal duties when it comes to allergens. Natasha’s Law (introduced in October 2021 after the tragic death of Natasha Ednan-Laperouse) requires all food businesses to provide accurate allergen information. The Food Information Regulations require restaurants, cafés, and takeaways to have systems in place to handle allergy declarations properly.

When a business fails in these duties and you suffer a reaction as a result, that’s not just bad luck — it may be negligence. And negligence means you might be entitled to compensation.

When You Can Claim Compensation for Shellfish Allergy

Not every allergic reaction leads to a compensation claim. But many do — especially when someone else’s negligence caused or contributed to your reaction.

You may have a claim if:

• A restaurant served you food containing shellfish after you declared your allergy
• A takeaway used the same fryer for your chips and someone else’s prawns
• A hotel buffet didn’t label dishes properly and you unknowingly ate something containing shrimp paste
• A work canteen failed to warn you that the curry contained shellfish
• A pre-packaged food item didn’t list crustaceans on the label (or the label was wrong)
• A wedding or event caterer ignored the dietary requirements you’d provided

The key factors are: you declared your allergy (or it should have been obvious from labelling), the business failed to protect you, and you suffered harm as a result.

We’ve helped people with shellfish allergy claims in all these situations. Each one is different, but the principle is the same: if a business was negligent, they’re responsible for the consequences.

One thing we hear time and again is that people struggled to find a solicitor willing to take on their allergy claim. Many firms simply don’t understand these cases — or don’t think they’re worth the effort. We disagree. If a business failed in their duty to protect you, that matters.

Evidence You’ll Need

If you’re thinking about making a claim, gathering evidence early makes everything easier. Here’s what helps:

• Medical records — GP notes, hospital discharge letters, ambulance records if you called 999
• Proof of the meal — Receipt, bank statement, online order confirmation
• Photos — Of the food, the menu, the allergen information (or lack of it)
• Witness details — Anyone who saw you declare your allergy or witnessed your reaction
• Your allergy history — Previous diagnosis records, prescriptions for adrenaline

Don’t worry if you don’t have everything — we can help gather what’s needed. But the more you can preserve early on, the stronger your claim.

Compensation Amounts

How much compensation you might receive depends on several factors: the severity of your reaction, how long your symptoms lasted, whether you were hospitalised, and any ongoing effects on your health or quality of life.

For most allergic reaction claims, compensation typically ranges from £1,500 to £3,500 for straightforward cases. More severe reactions — particularly those involving anaphylaxis, hospitalisation, or lasting psychological effects — can result in significantly higher awards.

Every claim is different. We can give you a clearer indication of what to expect once we understand your specific situation. For more detail on how compensation is calculated, see our allergy claims hub.

How the Claims Process Works

Making a compensation claim doesn’t have to be complicated. Here’s the typical process:

1. Free initial consultation. We’ll talk through what happened, assess whether you have a valid claim, and explain your options. No obligation, no pressure.

2. Investigation. If we take on your claim, we gather evidence, obtain medical records, and build your case.

3. Letter of claim. We formally notify the restaurant or business (and their insurers) that you’re claiming compensation.

4. Negotiation. Most claims settle without going to court. The other side’s insurers make an offer, we negotiate, and we advise you on whether to accept.

5. Settlement. Once we reach an agreement, compensation is paid — typically within 2-6 months of starting the process.

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