scientists do not yet thoroughly understand just how the body of an individual becomes sensitive to a substance that is harmless or even wholesome for the average person. milk, wheat, and egg, for example, rank among the most healthful and widely used foods. Yet these foods can cause persons sensitive to them to suffer greatly. at first, the body of the individual is not harmed by coming into contact with the substance. after a varying interval of time, usually longer than a few weeks, the body becomes sensitive to it, and an allergy has begun to develop. sometimes it's hard to fugure out if you have a food allergy, since it can show up so many different way. your symptoms could be caused by any other problems. you may have rashes, hives, joint pains mimicking arthritis, headaches, irritability, or depression.