A shellfish allergy is an immune system reaction that occurs when the body mistakenly identifies proteins found in shellfish as harmful. This allergy can cause symptoms such as hives, swelling, stomach pain, and difficulty breathing. It is one of the most common food allergies, particularly in adults. There are two main categories of shellfish: crustaceans (like shrimp, crab, and lobster) and mollusks (such as clams, oysters, and squid). People allergic to one type of shellfish may also react to others, so it is important to avoid all shellfish products. Diagnosis typically involves allergy testing and careful dietary management.