Shellfish

Shellfish as a Food Allergen in the Human Body

Shellfish allergy is a common IgE-mediated food allergy that occurs when the immune system mistakenly identifies certain proteins in shellfish as harmful and triggers an allergic response. Shellfish allergies can be severe and often persist for life.

Types of Shellfish Allergy

Shellfish allergy is divided into two main categories:

Shellfish Category	Examples	Cross-Reactivity Risk
Crustaceans	Shrimp, Crab, Lobster, Crawfish	Very high (most people allergic to one crustacean react to others)
Mollusks	Clams, Mussels, Oysters, Scallops, Squid, Octopus, Snails	Lower than crustaceans, some people react to both

Crustacean allergy is more common and severe than mollusk allergy.

Being allergic to crustaceans does not always mean being allergic to mollusks, but cross-reactivity is possible.

How Shellfish Allergy Develops in the Immune System

Sensitization Phase (First Exposure)

• Shellfish proteins enter the body through ingestion, inhalation (steam, cooking fumes), or skin contact.
• Dendritic cells in the gut-associated lymphoid tissue (GALT) or respiratory tract present these proteins to naïve T-helper (Th2) cells.
• Th2 cells activate B-cells to produce IgE antibodies specific to shellfish proteins.
• IgE binds to mast cells and basophils, “sensitizing” them for future exposure.

Allergic Reaction Phase (Subsequent Exposure)

• Upon re-exposure, shellfish allergens bind to IgE-coated mast cells, triggering mast cell degranulation.
• Histamine and inflammatory mediators (e.g., leukotrienes, prostaglandins) are released.
• This results in a rapid immune response causing allergy symptoms.

Major Allergenic Proteins in Shellfish

Shellfish allergy is primarily caused by Tropomyosin, a heat-stable muscle protein found in both crustaceans and mollusks.

Allergen Name	Protein Family	Function	Cross-Reactivity	Heat Stability
Tropomyosin (Pen a 1, Cha f 1, Pan s 1)	Muscle Protein	Muscle contraction	High (across all shellfish species)	Very heat-stable (remains allergenic after cooking)
Arginine Kinase (Pen m 2, Cha f 2)	Enzyme	Energy metabolism	Moderate (cross-reacts with insects and dust mites)	Somewhat heat-stable
Myosin Light Chain (Pen m 3, Cha f 3)	Muscle Protein	Muscle contraction	Moderate	Heat-stable
Hemocyanin	Oxygen Transport	Similar to hemoglobin in vertebrates	Cross-reacts with dust mites, cockroaches	Heat-sensitive

Tropomyosin is the primary allergen in shellfish allergy and is highly conserved across most shellfish species, meaning individuals allergic to one type of shellfish are likely allergic to others (high cross-reactivity).

Symptoms of Shellfish Allergy

Mild to Moderate Symptoms

• Oral Allergy Symptoms: Tingling, itching, or swelling of the lips, tongue, and throat.
• Skin Reactions: Hives (urticaria), eczema, or redness.
• Gastrointestinal (GI) Issues: Nausea, vomiting, abdominal pain, diarrhea.
• Respiratory Symptoms: Nasal congestion, sneezing, wheezing, difficulty breathing.

Severe Symptoms (Anaphylaxis)

• Swelling of the throat (angioedema), difficulty breathing.
• Drop in blood pressure, dizziness, fainting (anaphylactic shock).
• Severe wheezing or persistent coughing.
• Loss of consciousness (life-threatening reaction).

Even small amounts of shellfish can trigger severe reactions in highly allergic individuals, including airborne exposure from cooking vapors.

Cross-Reactivity in Shellfish Allergy

Shellfish allergy is linked to cross-reactivity with certain other allergens due to shared proteins.

Cross-Reactive Group	Examples	Cross-Reactive Protein
Crustaceans & Mollusks	Shrimp, Crab, Lobster, Clams, Mussels, Scallops, Squid	Tropomyosin
Insects	Cockroaches, Dust Mites, Grasshoppers	Tropomyosin, Arginine Kinase
Fish (Low Cross-Reactivity)	Salmon, Tuna, Cod	No significant cross-reactivity

Some shellfish-allergic individuals also react to dust mites and cockroaches due to Tropomyosin cross-reactivity (Shellfish-Mite Syndrome).

Shellfish Allergy Diagnosis

Skin Prick Test (SPT)

• A small amount of shellfish extract is applied to the skin.
• A positive reaction (redness, swelling) indicates sensitivity.

Serum-Specific IgE Test

• Measures IgE antibodies against shellfish allergens in the blood.
• High IgE levels indicate a stronger allergic response.

Oral Food Challenge (OFC)

• Gold standard test where small amounts of shellfish are consumed under medical supervision.
• Used when skin and blood tests are inconclusive.

Component-Resolved Diagnostics (CRD)

• Identifies specific allergenic proteins (e.g., Tropomyosin) in shellfish allergy.
• Helps determine cross-reactivity risk and reaction severity.

Managing Shellfish Allergy

Strict Avoidance of All Shellfish

• Avoid all crustaceans and mollusks if cross-reactivity is present.
• Be aware of shellfish-based ingredients:
  • Fish sauce (sometimes contains shellfish)
  • Worcestershire sauce (may contain anchovies)
  • Bouillabaisse (shellfish-based soup)
  • Imitation crab (surimi, often mixed with shellfish)
  • Omega-3 supplements (may be derived from shellfish)

Cross-Contact Risks

• Shared frying oil (e.g., fries cooked in oil that also fried shrimp).
• Grills and utensils used for cooking shellfish.
• Airborne exposure in seafood restaurants (cooking vapors can trigger reactions).

Emergency Treatment

• Epinephrine (EpiPen®) for severe reactions (anaphylaxis).
• Antihistamines (e.g., Benadryl, Zyrtec) for mild reactions.
• Corticosteroids to reduce prolonged allergic inflammation.

Conclusion

• Shellfish allergy is an IgE-mediated reaction, with Tropomyosin as the primary allergen.
• Symptoms range from mild oral itching to severe anaphylaxis.
• Shellfish allergy has high cross-reactivity among different shellfish species.
• Strict avoidance, label reading, and emergency preparedness (EpiPen®) are crucial for managing shellfish allergy.