An allergy to shellfish is an overreaction by your immune system to certain proteins found in shellfish. When you eat shellfish, your body mistakenly thinks you're doing something dangerous. Its reaction is to release an antibody to fight off the shellfish allergen. The antibody then sends out a bunch of chemicals, including histamine, to attack the shellfish protein. Your reactions to those chemicals are the unpleasant symptoms of an allergy to shellfish.
Generally, shellfish are divided into two different categories: crustaceans and mollusks. Crustaceans include crab, lobster, shrimp and crawfish; mollusks encompass abalone, clams, mussels, oysters, calamari, escargot, octopus and scallops. Some people are allergic to both categories of shellfish, while others are only allergic to certain shellfish. Generally, people with shellfish allergies try to avoid both groups, just to be safe. Not only do they avoid shellfish themselves, but they also avoid other foods that are cooked in the same oil or cookware as shellfish, along with the steam of cooking shellfish and bouillabaisse.