Two Types of Shellfish That Can Cause Allergic Reaction


Unlike many allergies that appear in childhood, allergic reaction to shellfish usually presents itself for the first time in adulthood. In fact, shellfish allergies are one of the most common to affect adults. Usually, people have an allergic reaction to shellfish such as shrimp and crab. But did you know that there are actually two types of shellfish that can cause allergic reaction?

Shellfish are divided into two categories, Crustacea and Mollusks.  Shellfish like shrimp, lobster, crab and crawfish are categorized as Crustacea. But shellfish like mussels, clams, oysters, scallops, abalone, octopus and squid (calamari) are classified as Mollusks.

It is possible for a person to be allergic to one type of shellfish but still be able to eat items from the other category. However, most people are allergic to both types, so physicians tend to advise strict avoidance of all shellfish to be safe.

Major Allergen in Shellfish

Many people who have a shellfish allergy have also been advised to be cautious of iodine.  While iodine is found in shellfish, iodine allergy is not related to shellfish allergy.  The main allergen in shellfish is a muscle protein called tropomyosin, which is found in both crustaceans and mollusks. Other minor allergens have also been identified in the two types of shellfish, but the most studied is the major allergen tropomyosin.

The structure of the allergen tropomyosin found in crustaceans is different than that found in mollusks. What that means is that a person who is sensitive to the allergen in one type of crustacean, like shrimp, will be highly likely to have a reaction to other crustaceans, like crab and lobster, but not necessarily to a mollusk, like abalone. Likewise, a person who is sensitive to the allergen in one type of mollusk, like squid, will be highly likely to have a reaction to other mollusks, like abalone and scallops, but not necessarily to a crustacean, like crawfish.

This difference can explain why some people will have an allergic reaction when they eat crustaceans but not when they eat mollusks and vice-versa. But, it is important to remember that an allergic reaction to shellfish is often very serious and is one of the main causes of anaphylaxis triggered by an allergen. So be sure to discuss with your allergist whether your allergy is to one, or both, types of shellfish and whether you should be eating either.

Common Foods That Contain Shellfish

Even though identifying and avoiding whole shrimp, scallops, crab or calamari may be relatively simple, shellfish can also be found hiding out in foods you might not expect. Some common foods that contain one or two types of shellfish include:

  • Bouillabaisse

  • Gumbo

  • Jambalaya

  • Cioppino

  • Fish stock

  • Imitation Seafood

  • Paella

  • WorcestershireSauce

  • Egg Rolls

  • Fish Sticks

  • Glucosamine

  • Chondroitin


Shellfish Allergies and Food Labeling

As a person dealing with food allergies, reading food labels has no doubt become second nature.  You are aware of the food labeling laws provided by FALCPA, the Food Allergen Labeling and Consumer Protection Act of 2004.  You know how to spot a potential allergen hidden in food products and how to avoid them.  However, mollusks are not considered major allergens under FALCPA and thus may not be disclosed on the product label in the same way as other allergens, if at all.

Under FALCPA, any food items included in one of the eight major food allergens must be clearly listed on the label in plain language by their common name.  The name must clearly appear either in the ingredient list or in a “contains” or “may contain” statement.   Crustaceans are considered one of the eight major food allergens.

But since mollusks are one of the two types of shellfish not considered an allergen for the purposes of FALCPA, any type of mollusk found in a particular food item need only be listed by its common name, without special attention.

Although many allergies can be outgrown, an allergy to shellfish is considered lifelong. It’s important to avoid all shellfish if you have been diagnosed with a shellfish allergy. To be safe, do not eat either type of shellfish without first consulting your doctor.

Do you have an allergy to one or both of the two types of shellfish? Want to share any label reading tips you’ve developed to identify mollusks hidden in food products? Please let us know in the comment section below.