Why do people get allergies?

Allergies are caused by your body's immune system over-reacting to contact with foreign substances like pollen, dust, pet dander, certain foods, or chemicals. Your immune system is designed to defend your body from harmful invaders, but in people with allergies, the immune system tries to fight off substances that can't harm the body. In a way, the system works too good. You want your body to fight off viruses and bacteria, but you don't need it to come to your defense for dust, pollen, or pet dander. Your immune system remembers these foreign substances it thinks are harmful and reacts the same way each time you're exposed to them. It's not clear why some people have allergies and some don't, but allergies do seem to be hereditary. If your parents or relatives have allergies, you have a pretty good chance of being allergic. As time goes on, more and more people are reporting allergy problems, and researches think that air pollution and recycled indoor air may be contributing factors. Also, better sanitation and the use of antibiotics may make our immune systems react more easily to harmless invaders because it's not as busy fighting off genuine infections.