Re: Why does all fish EXCEPT salmon give me hives?

Your question is very interesting. I did some medline searching through the medical literature (ussing the website http://www3.ncbi.nlm.nih.gov/entrez/ query.fcgi), but couldn't find an example of an allergy like yours.

Many people with seafood allergies will be allergic to more than one type of seafood, due to the fact that different fish have muscle proteins which are very similar. If your allergic response (being mediated by immunoglobulin E (IgE) molecules in your blood which recognize the fish protein) happens to be able to see the protein from different fish species, you'll get the same allergic response to any of these fish. It sounds like in your case you don't recognize the salmon version of whatever protein you are allergic to, but you do recognize this allergen from other species of fish.

The reason you break out in hives is because your body has some IgE which binds to the fish protein whenever you eat it. This complex then stimulates a type of cell (called a mast cell, abundant in your skin) to secrete histamine. Histamine then acts on the blood vessels in your skin to leak a bit, causing you to get hives and also itch. Histamine can also cause even more seriuos problems, like airway restriction and problems with your circulation. If you ever feel any of these symptoms (difficulty breathing, fainting), seek medical help immediately.

Taking an antihistamine like benadryl works because the benadryl will block the action of the histamine molecules. It is best, as you said, to take the benadryl before being exposed to the allergen, to completely block the histamine actions.

I should also say that allergies like this can sometimes get worse over the years, with repeated exposure to the allergen. You should be careful, and if you see your reaction getting worse over time, you may want to stop eating fish altogether.

Hope this helps.