Whats relatinship between iodine and shellfish allergy?

Same thing: Fish and shellfish have a tendency to concentrate iodine in their flesh. If you are allergic to iodine, often you will have a cross reactivity when you consume fish or shellfish. My wife, in fact, has this problem, and while she can eat fish from southern waters, she reacts poorly when she eats halibut caught in alaska. Particularly from the largest/oldest fish.