Shellfish allergy is an abnormal immune system response to specific proteins found in marine animals like crustaceans and mollusks. This reaction involves the production of Immunoglobulin E (IgE) antibodies, which trigger the release of chemicals leading to allergic symptoms. As one of the most common food allergies worldwide and a frequent cause of severe reactions, it represents a significant public health concern. Understanding the commonality and seriousness of this condition is important for diagnosis and strict dietary management.
Prevalence Statistics and Demographics
Shellfish allergy is one of the most common food allergies affecting adults, with an estimated global prevalence of about 3%. In the United States, self-reported allergy affects approximately 2.9% of adults, though physician-confirmed cases are closer to 1.2%. This discrepancy suggests that true clinical prevalence may be overestimated due to self-diagnosis or misdiagnosis.
This condition frequently begins in adulthood, often making it the most common food allergy to develop later in life. Approximately half of all adults with the allergy report developing it during their adult years, with the mean age of diagnosis around 28. Demographic studies show that non-White individuals are more likely to report a shellfish allergy than their White counterparts. Adults residing in ocean-adjacent counties also have elevated odds of having the allergy compared to those in non-coastal areas.
Distinguishing Crustacean and Mollusk Allergies
The term “shellfish” encompasses two distinct categories: crustaceans and mollusks. An allergic reaction to one does not automatically mean a reaction to the other. Crustaceans include animals such as shrimp, crab, and lobster, which possess segmented bodies and hard outer shells. Mollusks are soft-bodied invertebrates like clams, oysters, mussels, scallops, and squid.
Allergy to crustaceans is generally more common and is often associated with more severe symptoms than allergy to mollusks. In US adults, the estimated prevalence of crustacean allergy is 2.4%, which is higher than the 1.6% estimated for mollusk allergy. Individuals allergic to one species of crustacean, such as shrimp, have a high likelihood of reacting to other crustaceans because of the high similarity in their allergenic proteins.
Cross-reactivity between the two groups is not guaranteed, meaning many people allergic to a crustacean can safely eat mollusks. However, this cross-reaction is common; some studies suggest that up to 75% of people with a crustacean allergy may also react to mollusks. Individuals with a confirmed allergy to one group must consult a specialist to determine their risk profile for the other.
The Underlying Allergen and Clinical Severity
The primary reason for the commonality and cross-reactivity within shellfish is the major allergenic protein, Tropomyosin (TM). This muscle protein is highly conserved across invertebrate species, including crustaceans and mollusks, causing the immune system to recognize similar structures in different foods. Tropomyosin is also highly stable, retaining its allergenic structure even after high-heat cooking or the digestive process.
This stability and ubiquity contribute to the clinical severity associated with shellfish allergy. The condition is frequently linked to severe allergic reactions, including a high risk of anaphylaxis. Anaphylaxis is a rapid, life-threatening reaction that may involve symptoms like throat swelling, severe difficulty breathing, and a dangerous drop in blood pressure.
Shellfish is one of the most frequent food triggers for anaphylaxis in adults, often necessitating the use of an epinephrine auto-injector. The structural similarity of tropomyosin also leads to cross-sensitization with non-food allergens, such as those found in house dust mites and insects. This means some individuals may develop an IgE sensitivity to shellfish proteins without ever having consumed the food.