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Overview

An allergy is an abnormal immune response to a substance that does not cause a reaction in most people. Any substance that causes an allergic reaction is referred to as an allergen. When a person who is allergic comes in contact with an allergen, his or her immune system responds to the allergen as if it were a harmful substance, causing an allergic reaction.

Several different substances can cause allergies. Hay fever is a type of outdoor allergy caused by plant pollen (e.g., ragw...Read the full article