Conditions Dictionary

An allergen is a usually harmless substance capable of triggering a response that starts in the immune system and results in an allergic reaction. 

For instance, if you have an allergy to pollen, your immune system identifies pollen as an invader or allergen. The immune system responds by releasing chemicals that typically cause symptoms in the nose, throat, eyes, ears, skin or roof of the mouth.

In addition to pollen, other common allergens include dust mites, animal dander, mold, medications, insect venoms and various foods.

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AAAAI - American Academy of Allergy Asthma & Immunology