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How to Find Out if You’re Allergic to Shellfish

Picture the scene – you’re sitting down for a tasty restaurant meal with friends. You order crab. The waiter brings it to your table. It looks delicious and you tuck in eagerly, scooping the juicy flesh from the shell. Ten minutes later you start to feel uncomfortable and not long after that you’re so ill you have to go home.

Welcome to the world of shellfish allergies.

What Is A Shellfish Allergy?

Shellfish are one of the many food groups to which some people can develop allergic reactions. The specific trigger is types of protein within the fish.

The category of shellfish extends beyond crab of course. Biologically, crabs are crustaceans, and other types commonly seen on menus in seafood restaurants include:

  • Lobster
  • Shrimp
  • Crayfish
  • Prawn

And then there are mollusks. For example:

  • Oysters
  • Scallops
  • Squid
  • Clams

Crustaceans and mollusks each contain different types of protein and either – or both – may trigger reactions in susceptible individuals.

Like other allergies, reactions to shellfish vary. Some people react to any type and some to only one or two. Some sufferers experience only mild symptoms, while others require medical assistance for reactions that can be life-threatening.

Despite the fact that shellfish do not make up a large part of most people’s diets, shellfish allergies are amongst the most commonly occurring – alongside milk, eggs, soya and peanuts. You can also check out our article on the common symptoms of fish allergies.

Allergic reactions are caused by a badly ‘wired’ immune system, which routinely mistakes otherwise harmless foods for disease-causing organisms, or pathogens. When it detects a substance to which they have been sensitised, the immune system releases antibodies, the neurotransmitter histamine and other substances, and it’s this response that triggers the symptoms of an allergic reaction.

That much is understood, but there is still no consensus about why immune systems become sensitised to otherwise harmless substances. 

A variety of factors have been implicated, including infectious illness, industrial and urban environments and chronic stress. Genetics are often a major influence: a tendency for allergies can and does run in families. 

Another popular theory is the so-called ‘hygiene hypothesis’. This is the idea that our immune systems evolved to function with a certain level of exposure to microorganisms. Initially, these were identified as microbes and pathogens in general, but these days scientists believe specific microorganisms are key: species which have evolved within human beings for millennia. 

According to this theory, such exposure not only trains the immune system to respond to its environment in an appropriate way, it also keeps the system occupied. But if an individual grows up in an excessively hygienic environment, they may not have been exposed to a sufficient quantity or variety of microbes to properly prime their immune systems. Then, as they grow older, their underfunctioning system may then begin to misfire, triggering in response to foods and/or other harmless substances that the child may have come into contact with.

Research has shown that allergies are significantly more common in the industrialised world, adding weight to the hygiene hypothesis.

Common Shellfish Allergy Symptoms

As we saw above, the particular mix and severity of allergic reactions to shellfish will vary between individuals. Let’s take a look at some of the most frequently seen.


Hives, also known as urticaria, is a rash consisting of raised welts or bumps on various areas of the skin. Sometimes hives sting or itch. 

Hives are unpleasant but typically short lived. Some shellfish allergy sufferers also experience atopic dermatitis, also known as atopic eczema: a similar inflammation of the skin which can be triggered by allergies.


Sudden swelling is one of the more alarming allergic reactions. With food allergies this typically occurs in those parts of the body that have come into contact with the allergen – i.e. the tongue, throat and lips. Sometimes the face is also affected. 

Swelling is caused when, in an attempt to expel the allergen, the immune system releases the inflammatory chemical histamine. 


Wheezing is essentially obstructed breathing. We wheeze if our airways have become restricted or obstructed in some way: a frightening sensation that will be familiar to anyone who suffers from asthma. People with shellfish and other food allergies may wheeze if their throat is affected after exposure.


An allergic reaction to shellfish may bring on dizziness or lightheadedness, or even cause fainting. Dizziness is caused by the disruption of normal blood flow during an intense allergic reaction.

Shellfish Allergy FAQs

Is a shellfish allergy serious?

Most allergic reactions to shellfish are mild, but a proportion can be more severe, so it’s important to ensure you’re properly tested and have access to medical treatment if required.

How Do You Avoid Shellfish?

Unlike some other foodstuffs, shellfish is not too difficult to avoid if you have an allergy. Shellfish is not widely used as an ingredient in processed foods, although you may find it in some brands of salad dressing and sauce – for example steak sauce and Worcestershire sauce. Shellfish extracts are also sometimes used in Chinese foods like egg rolls and fried rice.

If you’re very sensitive, you may be at risk of cross contamination from fresh seafood in supermarkets or fish dishes in restaurants that also serve shellfish, so proceed with caution.

Some face cream and supplement brands contain shell extracts too, so it’s sensible to check the ingredient labels for such products. 

Order a shellfish allergy test

Allergies can be serious, so if you suspect you may be reacting to shellfish, it’s important to be tested as soon as possible. Your GP may be able to help but an alternative option is a home shellfish allergy test. After taking a blood sample via a quick and painless finger prick, you will be able to send this in for professional laboratory analysis. You can browse our food intolerance and allergy tests which analyse your blood for over 80 different food allergies and intolerances. 

Your detailed but easy-to-understand report will arrive a few days later. If you do have a shellfish allergy, you will then be able to seek appropriate medical advice and realign your diet with the reassuring certainty of avoiding those unpleasant symptoms and protecting your health. 

Order a food allergy test from Health Hub today.

Sian Baker

Medically reviewed by Sian Baker, Dip ION mBANT mCNHC – Written by Beth Giddings.

Updated on 1st December 2021