This is incredibly true. I had a friend who never had a problem with shrimp and at 27 started breaking out in hives everytime she ate shrimp. I wouldn't rule out pollutants (or even some individualized and specific reaction that occurs once or twice based on a multitude of other factors), but while I can only confirm this anecdotally and haven't looked into any science for it, it seems to come up often enough to be more than just an environmental or individual reason. I would really love to see any scientific reasoning for it with relation to shellfish as well as if there are any ethnic/age/gender specific factors for it to happen.