ysters are a mollusk. They are not a crustacean. Both crustaceans and mollusks can cause food allergy symptoms that range from the mild to the severe life threatening anaphylaxis. Those that are allergic to one type of shell fish may very well be able to eat the other type without any allergic reaction. People with allergies to dust mites and cock roaches have a greater likely hood have being allergic to crustaceans. That is because it is an IgE responsive allergy. IgE (immunoglobulin E) is the immune systems responsive antibody. Oyster allergy is less common then other forms of shell fish allergies.