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Shellfish allergy is one of the most common types of allergies. An estimated two per cent of the population of the United States suffers from allergies to shellfish, second only to peanuts. Unlike many allergies, you can’t outgrow shellfish allergy. It usually occurs first in adulthood, although many children suffer from it too. The allergy is most common in women and boys and is likely to run in families.

Symptoms of shellfish allergies can show up in minutes or they may take hours. The most common symptoms are hives (or urticaria) and swelling (angioedema). Hives are raised red weals that can appear anywhere on the body, but usually around the stomach, back, thighs, backside, limbs, and face. Those on the torso can be the size of dinner plates. Hives can be quite itchy. Angioedema is where blood cells produce fluid that builds up in the tissues under the skin and results in swelling. Common sites are the lips, tongue and throat, but also the hands and elsewhere.

Other symptoms of shellfish allergy can include tingling in or around the mouth or throat, chest tightness, nasal congestion, wheezing, breathing difficulties, light-headedness, dizziness, fainting, nausea, vomiting, stomach cramps, and diarrhea. Symptoms usually occur each time shellfish is eaten.

Potentially the worst symptom is an acute, whole-of-body allergic reaction. This is called anaphylaxis. The worst type is anaphylactic shock where a quick release of histamines and other mediators means fluid leaks into the skin. The blood vessels dilate, causing swelling, including to the throat. This constricts the airways. Blood pressure falls, and pulse can speed up or be faint. Any of the symptoms above can also be present. Disorientation can occur as the body goes into shock. Loss of consciousness and death are a risk, although luckily this is fairly rare.

Any of the shellfish can cause these symptoms. The two main types of shellfish are crustaceans and mollusks. Crustaceans include shrimps, prawns, crabs, crayfish, lobster, sea urchins, scampi, and langoustines. Mollusks include oysters, mussels, clams, squid (calamari), octopus, cuttlefish, scallops, abalone, cockles, whelks, periwinkles, conch, quahogs, limpets, and escargot (snails). A third type is echinoderms, such as sea urchins.

The two main groups of shellfish are biologically dissimilar and, while some people are allergic to one group and not the other, there is high cross-reaction between the two groups. If a person is allergic to a certain shellfish, they are 75 per cent likely to be allergic to others. Shrimps are worst as an allergenic, although there are cases where a person is allergic to some types of shrimps but not others. There is also a cross-reaction between shellfish and certain insects. No cross-reaction exists between shellfish and fish, although people can be allergic to both food types.

Shellfish allergies are caused by the proteins in the flesh. The main allergy-causing protein in shellfish is tropomyosin. The immune system produces antibodies against the allergen, causing the release of histamines and other chemicals. Tropomyosin is also found in some supplements, as well as in dust mites and cockroaches. Allergies can also be caused by the gelatin in the skin and bones of shellfish. Getting sick from shellfish might not be allergy but food poisoning from toxins or bacteria in the shellfish. Symptoms are similar, although the immune system isn’t involved with food poisoning.