Is It Possible To Be Allergic to Fish and NOT Shellfish?

Yes, you can be allergic to fish and not shellfish (and vice versa).

Why?

Because fish and shellfish are distinct groups of seafood, and allergies to them are caused by different proteins.

Fish allergies are typically triggered by proteins found in the muscle tissue of fish. Some of the common fish that most people may be allergic to include salmon, tuna, and cod.

On the other hand, shellfish allergies are usually divided into two groups: crustaceans (e.g., shrimp, crab, lobster) and mollusks (e.g., clams, mussels, oysters). The proteins that cause shellfish allergies are different from those found in fish. Fish and shellfish are rarely hidden on food labels; however, there are a few surprising places where fish and shellfish can be found.

For example, the following foods are some unexpected sources that may contain fish:
- Caesar dressing
- Worcestershire sauce
- Tomato sauce
- Imitation crab
- BBQ sauce
- Marshmallows

And some non-food products like:
- Lipstick
- Some fertilizers
- Dietary supplements
- Nail polish

Source: https://www.capitalconciergeallergy.com/post/shellfish-allergy-vs-fish-allergy