The body’s immune system normally fights infections. But when someone is allergic to shellfish, the immune system overreacts to proteins in the shellfish. Every time the person eats shellfish, the body thinks these proteins are harmful invaders.
The immune system responds by kicking into high gear to fight off the “invader.” This causes an allergic reaction in which chemicals like histamine are released in the body. In some cases, this reaction can even happen even if someone handles or breathes in airborne particles from shellfish.
The release of these chemicals can cause someone to have these symptoms:
itchy, watery, or swollen eyes
a drop in blood pressure, causing lightheadedness or loss of consciousness
People can have different reactions to different types of shellfish. It all depends on the person. Sometimes the same person can react in different ways at different times.
Shellfish allergies can cause a severe reaction called anaphylaxis. Anaphylaxis may begin with some of the same symptoms as a less severe reaction, but then quickly worsen, leading someone to have trouble breathing, feel lightheaded, or to pass out. If it is not treated, anaphylaxis can be life-threatening.