An allergy is a reaction of the immune system. The term allergy is also known as hypersensitivity. People who have allergies often are sensitive to more than one thing. Substances that often cause reactions are
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. Symptoms vary. Although allergies can make one feel bad, they are usually not fatal. However, a severe reaction called anaphylaxis is life-threatening.