"Shellfish" is a colloquial and fisheries term for aquatic invertebrates used as food, including various species of molluscs such as clams, mussels, oysters and scallops, crustaceans such as shrimp, lobsters and crabs, and cephalopods such as squid and octopus.

[6] Strong predictors for adult-persistence are anaphylaxis, high shellfish-specific serum immunoglobulin E (IgE) and robust response to the skin prick test.

[7][8] Food allergy symptoms usually occur within minutes after exposure but can be delayed to hours depending on the nature of the immune system response.

[9] Symptoms may include rash, hives, itching of mouth, lips, tongue, throat, eyes, skin, or other areas, swelling of lips, tongue, eyelids, or the whole face, difficulty swallowing, runny or congested nose, hoarse voice, wheezing, shortness of breath, diarrhea, abdominal pain, lightheadedness, fainting, nausea, or vomiting.

[9] Anaphylaxis occurs when IgE antibodies are involved,[10] and areas of the body that are not in direct contact with the food become affected and show severe symptoms.

The shellfish types causing clinical allergy are, in decreasing incidence: Once an allergic reaction has occurred it usually remains a lifelong sensitivity.

[7][8] Prevalence of seafood-induced adult asthma is reported as in the range of 7% to 36% (higher for crustaceans and lower for bony fish).

[8] The shellfish-induced health outcomes are mainly due to the protein tropomyosin causing an IgE mediated immune system response.

[15][16] Exposure to inhaled tropomyosins from dust mites is thought to be the primary sensitizer for shellfish allergy, an example of inhalant-to-food cross-reactivity.

[28] Two of the reviews postulate that exercise is not essential for the development of symptoms, but rather that it is one of several augmentation factors, citing evidence that the culprit food in combination with alcohol or aspirin will result in a respiratory anaphylactic reaction.

[33] This phase can either subside or progress into a "late-phase reaction" which can substantially prolong the symptoms of a response, and result in more tissue damage hours later.

[34] In the early stages of acute allergic reaction, lymphocytes previously sensitized to a specific protein or protein fraction react by quickly producing a particular type of antibody known as secreted IgE (sIgE), which circulates in the blood and binds to IgE-specific receptors on the surface of other kinds of immune cells called mast cells and basophils.

[33] Activated mast cells and basophils undergo a process called degranulation, during which they release histamine and other inflammatory chemical mediators called (cytokines, interleukins, leukotrienes, and prostaglandins) into the surrounding tissue causing several systemic effects, such as vasodilation, mucous secretion, nerve stimulation, and smooth-muscle contraction.

[35] In addition to IgE-mediated responses, shellfish allergy can manifest as atopic dermatitis, especially in infants and young children.

Tropomyosin, arginine kinase, myosin light chain and sarcoplasmic calcium-binding protein are widely present across shellfish species.

[6][17][18] Diagnosis of shellfish allergy is based on the person's history of allergic reactions, skin prick test and measurement of shellfish-specific serum immunoglobulin E (IgE or sIgE).

[41] Treatment for accidental ingestion of shellfish products by allergic individuals varies depending on the sensitivity of the person.

The Culinary Institute of America, a premier school for chef training, has courses in allergen-free cooking and a separate teaching kitchen.

Laws and regulations passed in the US and by the European Union recommend labeling but do not require mandatory declaration of the presence of trace amounts in the final product as a consequence of unintentional cross-contamination.

[56][60] The European Union requires listing for those eight major allergens plus molluscs, celery, mustard, lupin, sesame and sulfites.

[59] In Japan, a food-labeling system for five specific allergenic ingredients (egg, milk, wheat, buckwheat, peanut) was mandated under law on April 1, 2002.