Shellfish allergy

Shellfish allergy is a type of food allergy. Food allergy occurs when the body’s immune system wrongly identifies a food as a threat. When this happens, the body releases chemicals, such as histamine, in response. It is the release of these chemicals that causes symptoms.

If you react to one type of shellfish, it’s likely you’ll react to others in the same group. For example, if you react to crabs, it’s likely you’ll react to other crustaceans. You might react to shellfish in the other group as well, in this case molluscs.

You will also need to be careful of cross-contamination. At fish counters and markets different types of shellfish often touch each other whilst on display. If you eat a food contaminated by the shellfish you are allergic to, this could cause an allergic reaction.

• Scampi is the name given to a small type of lobster, which is a crustacean. When you buy scampi, always check to see if the company has used other shellfish, such as prawn. Pre-packed scampi should make the ingredients clear on the label. Occasionally, fish can be used to make scampi instead of shellfish.
• Oyster sauce is used to add flavour to some savoury dishes, especially in some multicultural cuisines such as Chinese noodle stir-fries, chow mein and beef with stir-fried vegetables.
• Fish sauce is commonly used in the UK and in multicultural cuisines. It can be made with shellfish or fin fish.
• Lancashire hotpot traditionally contained oysters, but a rise in price meant they are no longer commonly used.
• Stocks and soups. Some meals may seem to be safe options, but always check the ingredients in stocks and soups.

There is no evidence to suggest that people who are allergic to shellfish have a significantly higher risk of allergy to ‘fin fish’ such as cod, plaice, haddock, herring, trout, salmon and tuna.

Occasionally, people may be allergic to both shellfish and fin fish, but this is more likely to be a coincidence than cross-reactivity, which is where the proteins in one type of food are similar to the proteins in another. If you think you might be allergic to fish as well as shellfish, discuss this with your doctor.

Some medicines, supplements and cosmetics can contain extracts from the shells and skeletons of shellfish. You will need to avoid these.

• Glucosamine, used in the treatment of arthritis, can be taken from the skeletons of shellfish. Although one study found that glucosamine supplements from specific manufacturers posed no threat to people with shellfish allergy, it is sensible to use shellfish-free glucosamine, which comes from other sources.
• Chitin, found in shellfish shells, is used in commercial “fat absorbers” (weight loss supplements) such as Chitosan HD so you will need to avoid these. Some moisturisers also contain chitin from shellfish.
• Some calcium supplements contain ground oyster shells.

It is a common myth that people with shellfish allergy are likely to be allergic to radiocontrast dye containing iodine, which is sometimes used in medical procedures. This is incorrect and there is no link between shellfish allergy and symptoms caused by radiocontrast dye or iodine.

Shellfish do contain a small amount of iodine, but allergic reactions to shellfish are caused by a protein found in the muscles of shellfish, not the iodine.