The major allergens in fish are flesh proteins, which are very similar in all kinds of fish. This is why people allergic to cod tend to be allergic to fish such as hake, carp, pike, and whiting as well. As with shellfish allergy, sufferers are most often from regions of high fish consumption.
Fish allergy is sometimes confused with histamine (scombroid) fish poisoning, which is a toxic reaction to histamine in spoiled fish. The reaction is similar to an allergic reaction (swelling, hives, wheezing etc.)
Cooking appears to have little effect on fish allergens. Furthermore some fish allergic persons can get allergic symptoms due to the steam (airborne allergens) from cooking fish.