The meaty part
Your immune system reacts to proteins in the meat of the shellfish that causes allergic reactions.
Most people with "shellfish" allergy are allergic to crustaceans - shrimp, prawn, crab. Lobster, crayfish. Fewer are allergic to mollusks - clam, mussel, oyster, scallop, squid, octopus, abalone, cockles, snails. Even fewer are allergic to all shellfish. An allergist can help sort this out for you.