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.