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Home > Patient & Family Resources > Health Library > Allergic Reaction
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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.
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 help.
Allergies often occur along with other diseases, such as asthma, ear infections, sinusitis, and sleep apnea. For more information, see the topic Allergic Rhinitis.
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.
Allergies that occur for more than 9 months out of the year are called perennial allergies.
Year-round symptoms (chronic allergies) are most likely to occur from exposure to animal dander, house dust, or mold.
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:
You have answered all the questions. Based on your answers, you may be able to take care of this problem at home.
Symptoms of infection may include:
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 difficulty breathing can range from mild to severe. For example:
Certain health conditions and medicines weaken the immune system's ability to fight off infection and illness. Some examples in adults are:
Shock is a life-threatening condition that may quickly occur after a sudden illness or injury.
Adults and older children often have several symptoms of shock. These include:
Shock is a life-threatening condition that may occur quickly after a sudden illness or injury.
Babies and young children often have several symptoms of shock. These include:
Based on your answers, the problem may not improve without medical care.
Based on your answers, you may need care soon. The problem probably will not get better without medical care.
Based on your answers, you may need care right away. The problem is likely to get worse without medical care.
Based on your answers, you need emergency care.
Call 911 or 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.
Sometimes people don't want to call 911. They may think that their symptoms aren't serious or that they can just get someone else to drive them. Or they might be concerned about the cost. But based on your answers, the safest and quickest way for you to get the care you need is to call 911 for medical transport to the hospital.
You can use home treatment to relieve symptoms of:
Try a nonprescription medicine for the relief of itching, redness, and swelling. Be sure to follow the nonprescription medicine precautions.
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 reactions:
To prevent seasonal or year-round allergy reactions:
You can help your doctor diagnose and treat your condition by being prepared to answer the following questions:
Current as of:
June 26, 2019
Author: Healthwise StaffMedical Review: William H. Blahd Jr. MD, FACEP - Emergency MedicineKathleen Romito MD - Family MedicineAdam Husney MD - Family MedicineE. Gregory Thompson MD - Internal Medicine
Current as of: June 26, 2019
Author: Healthwise Staff
Medical Review:William H. Blahd Jr. MD, FACEP - Emergency Medicine & Kathleen Romito MD - Family Medicine & Adam Husney MD - Family Medicine & E. Gregory Thompson MD - Internal Medicine
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