Shrimp is consistently recognized as one of the most common causes of food allergy in adults and a frequent trigger for severe allergic reactions. A food allergy is a specific, reproducible adverse reaction to a food protein mediated by the immune system. Symptoms often manifest rapidly, frequently starting with a skin reaction like hives. Unlike some childhood food allergies, a shellfish allergy often persists throughout a person’s lifetime.
Recognizing Hives and Accompanying Symptoms
The most noticeable sign of a reaction to shrimp is often the sudden appearance of hives, medically known as urticaria. These are raised, intensely itchy welts on the skin that can appear anywhere on the body, though they frequently start around the mouth or face. Hives vary in size and shape, sometimes merging to form larger patches, and they usually appear within minutes to an hour of eating.
A shrimp allergy reaction often extends beyond the skin, involving a range of other systems. Symptoms can include a mild swelling of the lips, face, tongue, or eyelids, a condition called angioedema. Many people also experience digestive discomfort, such as nausea, vomiting, or diarrhea.
Respiratory symptoms may also occur, such as a runny nose, sneezing, or nasal congestion. These various symptoms represent a systemic response, where the immune system releases chemicals that affect different tissues throughout the body. While some reactions may be mild, any combination of symptoms should be taken seriously.
The Immune Response to Shellfish Proteins
The reaction is categorized as an Immunoglobulin E (IgE)-mediated rapid hypersensitivity reaction, meaning the response is swift and driven by specific antibodies. The primary culprit in a shrimp allergy is a muscle protein called tropomyosin, which is highly heat-stable and present across all crustaceans.
Upon first exposure, specialized immune cells produce large quantities of IgE antibodies specifically tailored to recognize this tropomyosin. These IgE antibodies then attach themselves to the surface of mast cells, which are immune cells located in tissues throughout the body, particularly the skin, lungs, and gastrointestinal tract.
The second time shrimp is consumed, the tropomyosin proteins bind directly to the IgE antibodies sitting on the mast cells. This binding acts as a trigger, signaling the mast cells to immediately degranulate and release a powerful cocktail of chemical mediators, most notably histamine. Histamine is responsible for the classic allergy symptoms; it causes blood vessels to dilate and leak fluid into surrounding tissues, resulting in the redness and swelling of hives and angioedema.
Immediate Treatment and When to Seek Emergency Care
For mild reactions, such as isolated hives or slight, localized swelling, an over-the-counter H1 antihistamine may be sufficient to block the effects of histamine and alleviate discomfort. After taking medication, it is important to monitor the reaction closely for several hours to ensure the symptoms do not progress.
A severe, life-threatening reaction is known as anaphylaxis. Anaphylaxis is indicated by symptoms affecting two or more body systems or any symptom indicating airway compromise or a drop in blood pressure. Signs include difficulty breathing, wheezing, a feeling of the throat tightening, persistent vomiting, or a sudden feeling of weakness or dizziness.
The most effective treatment for anaphylaxis is an injection of epinephrine, typically delivered via an auto-injector. Epinephrine works rapidly to relax the airway muscles, constrict blood vessels to raise blood pressure, and halt the release of further chemical mediators. If anaphylaxis is suspected, the auto-injector should be used immediately, and emergency services must be called simultaneously. Even if symptoms improve after the injection, the individual must be transported to an emergency room for observation, as symptoms can return hours later in a phenomenon called a biphasic reaction.
Diagnosis and Lifetime Management
After experiencing a reaction, confirming the allergy requires formal testing by an allergist. The most common diagnostic tools are the skin prick test and a specific IgE blood test. The skin prick test involves placing a small amount of shrimp extract on the skin and gently pricking the surface to check for a localized hive-like reaction.
The specific IgE blood test measures the level of shrimp-specific IgE antibodies circulating in the bloodstream. While these tests can indicate sensitization, a full diagnosis is based on a combination of the patient’s clinical history and the test results. A definitive diagnosis sometimes requires a medically supervised oral food challenge, though this is only performed in a controlled clinical setting.
Living safely with a shrimp allergy centers on strict avoidance of the allergen. This includes careful reading of food labels, as shrimp is considered a major food allergen and must be clearly disclosed on packaged foods in many countries. One must also be vigilant about cross-contamination in kitchens and restaurants, where shrimp may inadvertently contact other food items.
Shrimp is a crustacean, and allergy to it often means a high likelihood of cross-reactivity with other crustaceans, such as crab and lobster. This is because all these species share similar tropomyosin protein structures. Therefore, an allergist will typically recommend avoiding all crustaceans, and sometimes mollusks, to prevent a future allergic event.