Shellfish include two groups: crustaceans and mollusks. Crustaceans include lobsters, crabs, crawfish, prawns and shrimp. Mollusks include octopus, squids, abalones, clams, mussels, oysters, scallops and others.
Shellfish are not actually fish. The term is used by members of the fishing and culinary industries to refer to water-dwelling animals with an exoskeleton, that humans consume for food. Shellfish can be found in both fresh water and saltwater.
Shellfish allergies are among the most common types of allergic reactions found among humans. Such reactions are the result of the human immune system overreacting to certain proteins found in the food. The body begins to attack the proteins by releasing chemicals, which, in high doses, can lead to unpleasant or even severe reactions.