Shellfish allergies are an immune system response to certain proteins found in shellfish. Instead of recognizing these proteins as harmless, your body thinks they're dangerous and releases antibodies to eliminate them. The antibodies then send out a slew of chemicals to take care of the "invading" protein allergens. These chemicals are what can cause allergic symptoms, like a runny nose, itchy eyes, rash, hives, nausea, vomiting, diarrhea and difficulty breathing.
Shellfish are categorized into different groups, and the various groups contain different proteins that cause allergic responses. Crab, lobster, crayfish, prawns and shrimp are all crustaceans, whereas oysters, scallops, abalone, escargot and calamari all fall under the broad banner of mollusks. Some people can handle the proteins in one group but not the other, and some people have allergic reactions to all types of shellfish. Once you develop an allergy to the proteins in shellfish, you're unlikely to outgrow it.