Seafood allergies affect roughly 6.6 million Americans, according to the American Academy of Allergy, Asthma and Immunology. Like other food allergies, seafood allergies can be fatal if a person does not practice strict avoidance of all seafood products. Though most food allergies develop in young children and disappear as they grow older, seafood allergies frequently remain throughout adulthood. In fact, many people first develop seafood allergies as adults. Women are typically affected more than men.

The most serious type of allergic reaction to seafood involves a response known as anaphylactic shock. This potentially fatal condition can be triggered by even a very small amount of a seafood protein.

Three types of seafood can trigger allergic reactions:

• Fish. There are two main categories of fish: “bony fish” (e.g., salmon, cod, tuna) and “cartilaginous fish” (e.g., sharks). All bony fish share the protein parvalbumin, which is known to trigger allergic reactions. For this reason many people with seafood allergies are allergic to many different kinds of fish. It is not known whether cartilaginous fish also have parvalbumin. However, some people with fish allergies will be allergic to fish in both groups. Cod is the most widely reported fish allergy.
  
  
• Crustaceans. A large group that includes crabs, lobsters and shrimp. It is common for individuals allergic to this kind of seafood to react to all forms of crustaceans (and often mollusks as well).
  
  
• Mollusks. A broad group that includes mussels, oysters and clams. Most people who are sensitive to one type of mollusk are sensitive to them all, as well as all types of crustaceans.

An allergic reaction is triggered in sensitive individuals when a seafood protein (often parvalbumin) makes its way into the body. This process, called an allergic cascade, can begin when the patient:

• Eats seafood proteins
• Inhales proteins (e.g., from cooking seafood)
• Absorbs proteins through the skin (e.g., while handling seafood)

The allergic cascade is triggered when the immune system overreacts to a seafood protein, believing it is a dangerous substance. To combat the protein, the immune system releases IgE antibodies into the bloodstream. These antibodies interact with blood cells (mast cells and basophils) to trigger the release of histamines and other chemicals into the blood.

It is the histamines and other chemicals that stimulate allergy symptoms. By causing blood vessels to swell and individual cells to leak fluid, histamines produce allergy symptoms such as itchiness, rash, hives, stomach cramps, nausea and respiratory problems. Individuals who are sensitive to allergic seafood reactions can go into life-threatening anaphylactic shock, which involves breathing problems and lowered blood pressure.

In some cases, a reaction to fish may be caused by a toxin the fish ingested. Such a toxin may cause an allergic reaction, or it may cause food poisoning (which can be mistaken for an allergic reaction). Bacteria or viruses within seafood may also cause allergy-like symptoms – especially in areas with polluted coastal waters.

Raw fish tends to be the most dangerous form of seafood, provoking the strongest reactions in sensitive people. Cooking fish has proven effective at reducing the ability of the proteins found in fish to start an allergic reaction. Thoroughly cooking fish can make it possible for some mildly allergic individuals to consume fish without a reaction. However, only a physician can determine whether an allergic person can safely consume any type of seafood.

Seafood allergies tend to be more common in countries where seafood is a dietary staple and consumed regularly. In the United States, people are consuming more seafood than past generations. As a result, there has been an increase in the incidence of seafood allergies. Also, susceptibility to seafood allergies is believed to be genetic. Individuals with a personal or family history of any type of allergy (e.g., allergic rhinitis, eczema) are more likely to have a seafood allergy.