Home > Health & Wellness > Health Library > Allergic Reaction
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:
Seasonal allergies show up at the same time of the
year every year and are caused by exposure to pollens from trees, grasses, or
weeds. Hay fever is the most common seasonal allergy.
that occur for more than 9 months out of the year are called perennial
Year-round symptoms (chronic allergies) are
most likely to occur from exposure to
animal dander, house dust, or
Check your symptoms to decide if and when you should see a doctor.
Health Tools help you make wise health decisions or take action to improve your health.
Many things can affect how your body responds to a symptom and what kind
of care you may need. These include:
Certain health conditions and medicines weaken the immune system's ability to fight off infection and
illness. Some examples in adults are:
Symptoms of difficulty breathing can range from mild to severe. For example:
Symptoms of a severe allergic reaction
(anaphylaxis) may include:
A severe reaction can be life-threatening. If you have had a
bad allergic reaction to a substance before and are exposed to it again, treat
any symptoms as an emergency. Even if the symptoms are mild at first, they may
quickly become very severe.
Symptoms of infection may
You have answered all the questions. Based on your answers, you may be
able to take care of this problem at home.
Based on your answers, you may need care soon. The
problem probably will not get better without medical care.
Based on your answers, the problem may not improve without medical
Shock is a life-threatening condition that may occur quickly
after a sudden illness or injury.
Symptoms of shock in a child may include:
Shock is a life-threatening condition that may quickly occur
after a sudden illness or injury.
Symptoms of shock (most of which will be present) include:
Based on your answers, you need emergency care.
Call911or other emergency services now.
If you have an epinephrine shot, use it while you wait for help to arrive. Follow the directions on the label about how to give the shot.
Based on your answers, you may need care right away. The problem is likely to get worse without medical care.
You can use home treatment to relieve symptoms of:
For tips on how to treat dry and irritated skin, see the topic
Dry Skin and Itching.
For information on
how to treat an insect bite or sting, see the topic
Insect Bites and Stings and Spider Bites.
Call your doctor if any of the following occur during home treatment:
To prevent problems with severe allergic
To prevent seasonal or year-round allergy reactions:
Breast-feeding may prevent allergies. Breast-feed your baby
for at least 6 months if possible to boost his or her immune system. Feeding
only breast milk during the first 6 months of life may reduce the chances that your
child will develop
food allergies or may decrease the severity of your
child's allergies. For more information, see the topic
To prepare for your appointment, see the topic Making the Most of Your Appointment.
You can help your
doctor diagnose and treat your condition by being prepared to answer the
Current as of:
June 4, 2014
William H. Blahd, Jr., MD, FACEP - Emergency Medicine & H. Michael O'Connor, MD - Emergency Medicine
How this information was developed to help you make better health decisions.
To learn more, visit Healthwise.org
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