An allergy is an overreaction of the immune system to a substance that is harmless to most people. In someone with an allergy, the substance, or allergen, triggers the immune system to treat it as a harmful substance. The immune system releases chemicals that cause an allergic reaction.
Symptoms of an allergic reaction can be different for each person. These symptoms can include itchy eyes or itchy nose, sneezing, nasal congestion, throat tightness, trouble breathing or even shock (fainting or passing out). A severe allergic reaction is called anaphylaxis.
There are several different types of allergies: