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Quick Facts

Overview of Allergic Reactions

By The Manual's Editorial Staff,

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The immune system is your body's defense system. It helps protect you from illness and infection. The immune system usually reacts to and attacks bacteria, viruses, and cancer cells. An allergy is when your body's immune system reacts to something harmless, such as food, plants, or medicine.

What is an allergic reaction?

An allergic reaction is what happens when you touch, eat, or breathe something you're allergic to. Allergic reactions can be mild or severe.

  • Mild reactions are unpleasant and annoying

  • Severe reactions can be life-threatening

What causes an allergic reaction?

Doctors aren't sure why some people who are exposed to a substance become allergic to it and others do not.

  • Allergies seem to run in families

  • What you were exposed to and ate when you were a child may affect whether you get allergies

Common substances that trigger allergic reactions include:

  • House dust

  • Animal dander

  • Pollen (of trees, grass, weeds)

  • Molds

  • Certain foods

  • Certain medicines

What are the symptoms of an allergic reaction?

Mild allergic reactions may cause:

  • Watery, itchy eyes

  • Runny nose and sneezing

  • Itchy skin, sometimes with a rash (hives)

Severe allergic reactions (anaphylaxis) may cause:

  • Swollen eyes, lips, tongue, and throat

  • Wheezing and trouble breathing

  • Belly cramps, feeling sick to your stomach, and throwing up

  • Dizziness and fainting because of a drop in your blood pressure

  • Death

How do doctors tell if I have an allergic reaction?

Doctors can usually tell based on your symptoms and by examining you. However, it can be hard for doctors to tell exactly what you're allergic to. They may do skin tests or blood tests to find out.

How do doctors treat an allergic reaction?

For mild reactions, your doctor may give you:

  • Corticosteroid nose sprays

  • Antihistamines

  • Decongestants

  • Eye drops

For severe reactions, you may need:

  • A shot of epinephrine

  • Antihistamines and corticosteroids given by vein (IV)

  • Medicine and fluids given by vein to raise your blood pressure

  • Medicine to open your airway and help you breathe

  • Sometimes a breathing tube down your throat so doctors can put you on a ventilator to help you breathe

How can I prevent allergic reactions?

The most important ways to prevent allergic reactions:

  • Avoid the things you’re allergic to

  • If you can't avoid the things you're allergic to, ask your doctor about allergy shots

Avoiding an allergen may involve:

  • Stopping a medicine

  • Keeping pets out of the house or limiting them to certain rooms

  • Using high-efficiency particulate air (HEPA) vacuums and filters

  • Not eating a certain food

  • Removing or replacing items that collect dust, such as soft furniture, carpets, and knickknacks

  • Putting special covers on mattresses and pillows to keep out dust mites

  • Using synthetic-fiber pillows

  • Frequently washing bed sheets, pillowcases, and blankets in hot water

  • Frequently cleaning the house, including dusting, vacuuming, and wet-mopping

  • Using air conditioners and dehumidifiers in basements and other damp rooms

  • Getting rid of cockroaches

With allergy shots, the doctor gives you shots of the substance you are allergic to. At first the shots have only a very, very tiny amount of the substance. The amount is too small to cause a severe reaction. Then the doctor gives you shots that have more and more of the substance. That way, your body can become used to the substance and not react to it. Allergy shots don't always work. And when they do work, you may have to keep getting the shots.

Drugs Mentioned In This Article

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  • ADRENALIN