Is it possible to be allergic to only certain types of shellfish?
Crustaceans are the group of seafood responsible for generating the most reactions. Commonly eaten fish in the shellfish group include shrimp, crab, lobster, and crayfish (via Uptown Allergy and Asthma). Molluscs are another group somewhat similar to crustaceans, including scallops, clams, mussels, and oysters. A third group that stands out for its lack of shells are finfish like salmon, tuna, and halibut.
The diagnosis of a seafood allergy is determined by testing with an allergist. According to the American College of Allergy, Asthma & Immunology (ACAAI), you will be asked about foods eaten and symptoms experienced. Next, a blood or skin test will be administered. The skin prick reveals results within 30 minutes thanks to a small reaction on the skin, while a blood test will measure the antibodies present against the food in question, with results taking up to two weeks.
The ACAAI notes that you can be allergic to one group of fish and not the other. This variable is a double-edged sword since it means that you may be able to enjoy certain seafood despite having an allergy to another, but to do so you will need to determine whether or not you react to each group.
If you are allergic to any type of fish, the ACAAI recommends consulting an allergist to determine if it is safe to eat other types of fish. You should also be aware of the risk of cross-contamination, as different types of shellfish are often stored together in markets and restaurants.