An allergy is a reaction of the immune system to something that does not bother most other people. People who have allergies often are sensitive to more than one thing. Substances that often cause reactions are: pollen, dust, mold, food, insect stings and medicines. Scientists think both genes and the environment have something to do with it. Normally, your immune system fights germs. It is your body’s defense system. In most allergic reactions, however, it is responding to a false alarm. Allergies can cause a runny nose, sneezing, itching, rashes, swelling or asthma. Although symptoms can vary in severity and discomfort, a severe reaction called anaphylaxis is life threatening.