Oh yea, I have that allergy.
I ate shellfish all of my life until, at age 35, we were on a vacation here, on Cape Cod, searching for a house in the Boston area, preparing for our move from AZ. I ate a lobster roll and within 10 minutes was having a serious reaction, lips tingling, etc. I took my allergy meds and inhaler and never touched shellfish again. This was at the time I was having a lot of asthma/allergy issues. For awhile, I couldn't drink wine, either (sulfites). I can eat scallops, though.
Three years ago, when I was going through all of those medical tests, I had an abdominal CT scan. Within 10 seconds of the dye being injected, my right eye swelled shut and I had the hive the size of an egg on my wrist. Although I had no issues breathing, etc. I totally freaked out, so they shot me up with Benedryl and epinephrine and DH had to come get me at the ER. I kept telling them I didn't want the dye, as my mom had an iodine allergy. But no, they insisted.
I hope I never need stress test where they need to see your arteries, or any of the other things my DH has had.