I think I am having an allergic reaction to crab. How long do the hives last? From the reaction I have them all up my arms and on my back, and they still itch.
I have an allergy to just crab and even with the smallest amount i get itchy and break out in hives. For me, it depends on the amount of crab that i have eaten. The smallest amount brings on a reaction, but the more amount of crab i consume the worse the itching and the hives become. One time i had a bite of crab and had a reaction for 2-3 days. The next week I had the same amount again (don't ask me why) and my reaction was worse. I was burning, itchy and covered in hives for a week. Everything swelled up and I couldnt even put on shoes. So I would say it depends on the amount you had consumed.
I have had hives last for days. You should see a doc and get a prescription for steroids. Shellfish is one of those potentially life threatening allergens, with each reaction possibly being worse than the last.