An allergy is a type of immune response. The purpose of the immune system is to recognize harmful infection and to get rid of them. However, for some people, the immune system overreacts to ordinarily harmless substances like pet dander, pollen, dust, pollen from grass, trees and weeds.

Common allergic reactions include eczema, hives, hay fever, asthma, sneezing, a runny nose, itchy. In some people, severe allergies to environmental or dietary allergens or to medication may result in life-threatening anaphylactic reactions and potentially death.