Is there a reason why i am allergic to fish but not shellfish?

Fishy allergies: Fish and shellfish share few allergens. Usually we divide seafood allergies into three categories: crustaceans such as crab, shrimp and lobster, mollusca including clams, oysters and mussels, and finned fish such as salmon and tuna. Within each group people often react to many or all of the individuals. While you can be allergic to all three groups, there is little cross-reactivity between them.