I like Deb's advice about offering to bring something.
Well, I don't eat meat - but I never want anyone fussing about that *ever* -- I cannot stand to have that fussed about, bc honestly, I can always find stuff to eat at gatherings. I just don't want it to be a big deal.
But, I think an allergy/sensitivity is totally different - bc those things can make you miserable! So, if I was a hostess and someone let me know of an issue and offered to bring something or had some suggestions for me - I'd be HAPPY they spoke up.