A shellfish allergy is an immune system response to proteins found in aquatic animals, primarily crustaceans and mollusks. While many people allergic to one type of shellfish react to others, it is biologically possible to be allergic to lobster (Homarus americanus) but not crab. This difference in reaction occurs because the immune system selectively targets minute structural distinctions in the otherwise highly similar proteins found across these species.
The Primary Allergen in Crustaceans
The vast majority of allergic reactions to crustaceans, including lobster, crab, and shrimp, are caused by a single muscle protein called Tropomyosin (TM). This protein is a highly stable component of the muscle fibers, where it plays a role in contraction.
Tropomyosin is a relatively small protein, typically ranging from 35 to 38 kilodaltons (kDa) in size, and it forms a coiled-coil dimer structure. Because of its inherent stability, this protein is resistant to heat, digestion by stomach acids, and other food processing methods. This resilience means the allergenic protein remains intact after ingestion, increasing the likelihood of an immune system response. The immune system identifies this stable protein as a threat, triggering the production of IgE antibodies.
Why Selective Allergy Occurs
The phenomenon of selective allergy, where a person reacts to lobster but tolerates crab, relates directly to the concept of cross-reactivity. Crustaceans share a high degree of protein similarity; the amino acid sequences of Tropomyosin between lobster and crab can overlap by 91% to 100%. This high homology explains why most people with a crustacean allergy react to multiple species.
The slight differences in the protein structure, however, determine selective sensitivity. The immune system’s IgE antibodies recognize specific small regions on the allergen called epitopes, or antigenic sites. Although the overall Tropomyosin structure is nearly identical, minor variations in the amino acid sequence at these specific epitope locations can be enough for the IgE antibodies to bind to the lobster protein, while failing to bind to the crab protein.
This molecular distinction explains why a diagnostic test may show sensitization to the general crustacean protein, but a clinical reaction only occurs with one species. The difference is not in the type of protein, but in the subtle folding and sequence variations that exist even within closely related species.
Diagnosis and Risk Management
Diagnosing a selective allergy involves a multi-step approach, starting with a detailed patient history of reactions to specific shellfish types. Initial testing often includes skin prick tests (SPT) or specific IgE blood tests, which measure the presence of IgE antibodies against shellfish extracts or purified Tropomyosin. A positive result on these screening tests indicates sensitization, but it does not always confirm a clinical allergy.
Since these initial tests can yield false-positive results due to the high cross-reactivity of the Tropomyosin protein, the gold standard for definitive diagnosis is the oral food challenge (OFC). This procedure involves consuming small, increasing amounts of the suspected allergen under strict medical supervision to observe for any reaction.
For selective sensitivity, an allergist may use component-resolved diagnosis (CRD) to specifically look for IgE antibodies targeting the Tropomyosin from lobster, rather than a broad crustacean extract. Once a selective allergy is confirmed, the primary management strategy is strict avoidance of the confirmed allergen, such as lobster, while safely continuing to consume tolerated species like crab.
Patients must meticulously read all food labels, as the Food and Drug Administration (FDA) requires manufacturers to list major allergens like shellfish. Individuals with any diagnosed food allergy should always carry an epinephrine auto-injector, as this remains the only first-line treatment for anaphylaxis.