The key words are: "Plan Ahead"
Definitely mention any allergy that could cause a serious reaction. When you call to RSVP, explain to your host, “We’d love to come for dinner, but I have to tell you that Becky is deathly allergic to peanuts.” Dietary preferences such as vegetarianism or a low-carb diet should be handled differently. At large parties and buffets, where there’s usually a large variety of foods to choose from, make do with what is available. However, if the gathering is small, the dinner is in your honor, or you’re going to be an overnight houseguest, do mention your restrictions and offer to bring a dish to share. Say, “Thanks so much for the invitation. I should let you know that I’m a vegetarian. I’d love to bring a quiche if that’s okay with you.” This way, your host won’t waste time preparing the wrong food for you or have to trouble himself figuring out what type of dish would best suit your preferences.