Allergies are an overreaction of the body's natural defense
system that helps fight infections (immune system). The immune
system normally protects the body from viruses and bacteria by
producing antibodies to fight them. In an allergic reaction, the
immune system starts fighting substances that are usually
harmless (such as dust mites, pollen, or a medicine) as though
these substances were trying to attack the body. This
overreaction can cause a rash, itchy eyes, a runny nose, trouble
breathing, nausea, and diarrhea. An allergic reaction may not
occur the first time you are exposed to an allergy-producing
substance (allergen). For example, the first time you are stung
by a bee, you may have only pain and redness from the sting. If
you are stung again, you may have hives or trouble breathing.
This is caused by the response of the immune system. Most people
will have some problem with allergies or allergic reactions at
some point in their lives. Allergic reactions can range from
mild and annoying to sudden and