Allergic Reaction - Topic Overview
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
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.
Many 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 life-threatening. Most
allergic reactions are mild, and home treatment can relieve many of the
symptoms. An allergic reaction is more serious when severe allergic reaction
(anaphylaxis) occurs, when
allergies cause other problems (such as nosebleeds,
ear problems, wheezing, or coughing), or when home treatment doesn't
Allergies often occur along with other diseases, such as
sleep apnea. For more information, see the topic
There are many types of allergies.
Some of the more common ones include:
- Food allergies, which are more common
in children than adults. Food allergies are most common in people who have an
inherited tendency to develop allergic conditions. These people are more likely
to have asthma and other allergies. For more information, see the topic
- Medicine allergies. Many prescription
and nonprescription medicines can cause an allergic reaction. Allergic
reactions are common and unpredictable. The seriousness of the allergic
reaction caused by a certain medicine will vary.
- Allergies to insect venom. When you are stung by an
insect, poisons and other toxins in the insect's venom enter your skin. It is
normal to have some swelling, redness, pain, and itching at the site of a
sting. An allergic reaction to the sting occurs when your body's immune system
overreacts to the venom of stinging insects. For more information, see the
topic Allergies to Insect Stings.
- Allergies to animals, which are more likely to cause breathing problems than skin
problems. You may be allergic to your pet's dead skin (dander), urine, dried
saliva, or hair.
- Allergies to natural rubber (latex).
Some people develop allergic reactions after repeated contact with latex,
especially latex gloves.
- Allergies that develop from exposure to a
particular inhaled substance in the workplace. These are called
- Allergies to
cosmetics, such as artificial nails, hair extensions, and henna tattoos.