How common is it to develop an allergic reaction to food? if you never had allergies before.

Not rare: An allergic respose takes place when your immune system recognizes a foreign substance. Highly allergic people's immune system does this quickly. Another person may need to be exposed to the substance (food) hundreds of times before it reacts to it. Once it recognizes it as foreign it will repond instantanously every time it sees it. A never allergic respose turns into an always response.