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Information An allergy is an exaggerated immune response or
reaction to substances that are generally not harmful. Causes,
incidence, and risk factors: Allergies are relatively common.
Both genetics and environmental factors play a role. The immune
system normally protects the body against harmful substances,
such as bacteria and viruses. It also reacts to foreign
substances called allergens , which are generally harmless and
in most people do not cause a problem. But in a person with
allergies, the immune response is oversensitive. When it
recognizes an allergen, it releases chemicals such as
histamines. which fight off the allergen. This causes itching ,
swelling , mucus production, muscle spasms , hives , rashes ,
and other symptoms, which vary from person to person. Common
allergens include pollen, mold, pet dander, and dust. Food and
drug allergies are common. Allergic reactions can also be caused
by insect bites , jewelry, cosmetics, spices, an