Most seafood allergies stem from something the seafood ate, not the seafood itself. A tiny plankton like animal that some fish and shellfish ingest contains a toxin that makes humans break out and swell up like they had a severe rash. It happens to me every now and then. I still eat seafood, but try to stay with farm raised fish and shrimp, and crabs.........Less likely to make me look like a GEICO caveman the next day........
That may be, although I've been sick about 60 to 70% of the time I've eaten seafood...at this point the smell of fish reminds me of being sick, so I just pass on it. (no pun intended).