Shellfish

Shellfish allergy is one of the most common and most persistent food allergies in adults, often appearing for the first time in adulthood and rarely outgrown. It splits into two groups: crustaceans (shrimp, prawn, crab, lobster, crayfish) — more allergenic — and mollusks (clams, mussels, oysters, scallops, squid, octopus). Many people allergic to crustaceans tolerate mollusks, and vice versa, but assume cross-reactivity unless an allergist confirms otherwise.

The primary allergen in shellfish is tropomyosin, a muscle protein that's highly conserved across species. This is why shellfish allergy persists — repeated exposure doesn't induce tolerance the way some childhood food allergies resolve. Reactions can be severe and include anaphylaxis; carrying epinephrine and avoiding even small amounts is standard.

Tropomyosin and cross-reactivity

Tropomyosin from shellfish cross-reacts with tropomyosin in dust mites and cockroaches — which is why some shellfish-allergic patients also react to inhalant allergens, and why some dust-mite-allergic patients develop adult-onset shellfish allergy. Iodine and contrast-dye allergy is unrelated to shellfish allergy, despite the old myth.

Hidden sources

Hidden shellfish exposure: fish sauce, Worcestershire sauce, seafood-flavored chips and seasonings, imitation crab (which is often fish, but processed in shellfish facilities), some dietary supplements (glucosamine often from shellfish). Cross-contamination is high risk in seafood restaurants, Asian cuisines, shared fryers, and buffet settings — even airborne protein can trigger reactions in highly sensitive people.