Allergies to shellfish and scaly fish are the most common types of food allergies. When your body misidentifies the proteins in fish as harmful, it sends out an antibody called immunoglobulin E to attack the invading allergen. The antibody then releases other chemicals to combat the fish protein. You react to the chemicals coursing through your body by displaying various allergic symptoms.

The most common symptoms of a fish allergy appear on your skin; such symptoms include eczema, hives and swelling. Other common symptoms of a fish allergy include digestive problems like vomiting, nausea and diarrhea. Sometimes, respiratory symptoms accompany other symptoms, but difficulty breathing doesn't usually result from a fish allergy on its own. In some cases, people with fish allergies can suffer anaphylaxis when exposed to fish protein. Anaphylaxis is a potentially fatal condition in which you may suffer dizziness, difficulty breathing and even loss of consciousness. People who exhibit the signs of anaphylaxis must seek medical attention immediately.

Occasionally, people suffer from other reactions to fish that may be confused with allergies. For example, a common type of mild food poisoning called histamine fish poisoning, or HFP, is often mistaken for a food allergy. In HFP, slightly spoiled dark-meat fish like tuna, mackerel and bonito can cause nausea, vomiting, diarrhea, hives, itching, rash and a burning sensation in the mouth. Symptoms start a few minutes after you eat the contaminated fish, and they can last up to 24 hours. HFP also affects mahi-mahi, bluefish, sardines and yellowtail. You can't detect histamine poisoning by sniffing fish since the bacterial increase is minor. When the fish is caught, it's toxin-free, but the bacteria develop before it cooks, often due to improper refrigeration. Cooking doesn't destroy the bacteria. Another condition that's often mistaken for fish allergies is actually an allergy to a parasitic worm called Anisakis that's often found in fish. While it doesn't usually cause anaphylaxis, it can cause other allergic symptoms.