Causes

All food allergies are caused by an immune system overreaction. In shellfish allergy, your immune system mistakenly identifies a certain protein in shellfish as harmful, triggering the production of antibodies to the shellfish protein (allergen). The next time you come in contact with the allergen, your immune system releases histamine and other chemicals that cause allergy symptoms.

Types of shellfish

There are several types of shellfish, each containing different proteins:

• Crustaceans include crabs, lobster, crayfish, shrimp and prawn.
• Mollusks include squid, snails, clams, oysters and scallops.

Some people are allergic to only one type of shellfish but can eat others. Other people with shellfish allergy must avoid all shellfish.