So, just got back from the allergist. The conclusion in summary: we don't really know what caused the reaction.

He thought the crab cakes sounded like the most likely cause, due to the timing of when I ate them and when the symptoms began. Then he tested me for allergies to a bunch of different foods but none yielded a positive result. However, he told me that the tests for food allergies are less reliable than tests for things like mold and dander, and in fact the test for cashews was also negative even though I've had a clear reaction to them in the past. Also it's possible that different types of crabs have slight differences in protein structure and one could cause an allergy while the other doesn't.

So he recommended that I avoid crab and other shellfish for now. In a few months, I can try a nibble of someone else's crab cake. If I have no reaction, I can try a larger nibble at another time, and if that's okay I can have a whole bite. If there's no reaction to that, I can have a whole crab cake and I'll probably be okay.

In the meantime, he prefers Zantac as an antihistimine for this type of reaction, and he recommends I keep some on hand (and I can keep the Claritin around also -- no harm to use both) in case I have problems in the future. He's not a big fan of Benedryl unless I'm having trouble sleeping due to the itch (which definitely was the case with this recent attack).

I was prepared for negative test results, so I'm not really disappointed or surprised. Overall I liked the doctor a lot. This was my first visit with him. He's an older man (called me "young lady" twice, which is something I rarely hear these days ) but from talking to him I could tell he keeps on top of current research.

Some other interesting things he mentioned -- a large number of people who get hives have them due to an autoimmune response, not because of a specific allergy trigger. Food allergies are more likely to manifest within ~12 hours of eating the food, so he wasn't particularly interested in anything I'd had prior to the day the attack happened (the symptoms began around 7 pm). It's possible that the symptoms might return in a few days, because I'm still tapering off the prednisone and just as there is a 1-2 day lag for that drug to take effect, there is a 1-2 day lag for it to completely clear your body. So I should have Zantac handy for the next week or two, just in case.

I do love crab cakes and will miss eating them.

BTW I did not mention the possibility of pseudoallergy/histamine intolerance to the doctor, mostly because there were so many other things to discuss while I was there and I would like to focus on the crab thing for now. But I'm keeping it in the back of my mind for the future. For example in the week before this happened I changed two bike tires (latex exposure) and ate a tuna sandwich with avocado which has the same allergen in it as latex. In the future I'll try to be aware of those kinds of multiple exposures that might cause a cumulative affect. I did have the same sandwich with avocado yesterday for lunch, and have not had any ill effects.

I have to say, allergies suck, but on another level they are kind of fascinating.