I have issues with dairy and red food coloring. My DS has issues with dairy, gluten, nitrites/nitrates, red and blue food coloring, and antibiotics and hormones given to chickens. He needs all natural, organic eggs and chicken. I am pretty sure he reacts to many preservatives, too. We are still trying to narrow everything down.
A lot of people react to the artificial food dyes. Search them on google and you can find a lot of info. In fact, many of the dyes we still see in the US have been banned in Europe. You can read more here:
http://www.organicauthority.com/blog...thority.com%29
I hope your hives clear up soon and you find the root of your problem with the cake! Food reactions are no fun at all (and sometimes dangerous).