I too am allergic to shell fish... Not all of them, but some. Also from the iodine but I guess I'm not as allergic as some. Shrimp and oysters are the two things that really set me off... I can have one or two medium sized shrimps and be OK, but that third one and I swell up like a blow fish. Other stuff I can eat like, crab, lobster, clams it's weird... I just have to be careful about it. There are also a lot of medications out there that have various shellfish or fish ingredients that I have to be careful with.
So having faced this for as long as I can remember, I'm always very diligent about ingredients when cooking for others.