I know a lot of people who used to say that they don't have allergies suddenly get sneezing fits and rashes after exposure to things that were previously harmless. The refuse to believe that an allergy can be acquired, or better to say, manifested, later in life, even after repeated exposures that did not elicit a reaction. This is called adult-onset allergies. According to allergist Dr. Fred Virant:

[snip] genetics play a big role in determining when allergies will surface.

Fred Virant, M.D.: "There's probably seven or eight genes involved and obviously, that child that has more trouble has them all switched on."

Doctor Virant says adults who suddenly find themselves suffering from allergies only have one or two of those genes switched on. Often it's a simple change that triggers a reaction.

Fred Virant, M.D.: "They just got a new pet, or they've moved to a new area."

This also busts the myth that repeated exposure to an allergen-whether they are pollen, food or insect stings- will eventually make you "immune" and help you overcome your allergies. In fact, if you have the genetic predisposition to having allergies, repeated exposure will just increase the likelihood of your allergies manifesting.