Allergic Reaction

Written by Darla Burke | Published on July 11, 2012
Medically Reviewed by Brenda B. Spriggs, MD, MPH, FACP

What Is Allergic Reaction?

Your immune system is responsible for defending the body against bacteria and viruses. In some cases, your immune system will defend against substances that typically do not pose a threat to the human body. These substances are known as allergens, and when your body reacts to them, it causes an allergic reaction. Allergens that cause a reaction can come in contact with the skin, be inhaled, or be eaten.  Allergens can also be used to diagnose allergies and even be injected as form of treatment.

The American Academy of Allergy, Asthma & Immunology (AAAAI) reports that as many as 50 million people in the United States suffer from some type of allergic disease. (AAAAI)

What Causes an Allergic Reaction?

Doctors do not know why some people experience allergies. Allergies appear to run in families (inherited). If you have a close family member who has allergies, you will have a greater risk for developing allergies.

Although the reasons allergies develop are not known, there are some substances that commonly cause an allergic reaction. People who have allergies are typically allergic to one or more of the following:

  • pet dander
  • bee stings or bites from other insects
  • certain foods, including nuts or shellfish
  • certain medications such as penicillin or aspirin
  • certain plants
  • pollen

What Are the Symptoms of an Allergic Reaction?

The symptoms of an allergic reaction can vary from mild to severe. If you are exposed to an allergen for the first time, your symptoms may be mild. These symptoms may get worse if you are repeatedly exposed to the allergen.

Symptoms of a mild allergic reaction can include:

  • hives (itchy red spots on the skin)
  • itching
  • nasal congestion (rhinitis)
  • a rash
  • watery or itchy eyes

Severe allergic reactions can cause other symptoms such as:

  • abdominal cramping or pain
  • pain or tightness in the chest
  • diarrhea
  • difficulty swallowing
  • dizziness (vertigo)
  • fear or anxiety
  • flushing of the face
  • nausea or vomiting
  • heart palpitations
  • swelling of the face, eyes, or tongue
  • weakness
  • wheezing
  • difficulty breathing
  • unconsciousness

A severe and sudden allergic reaction can develop within seconds of being exposed to an allergen. This type of reaction is known as anaphylaxis and results in life-threatening symptoms, including swelling of the airway and the inability to breathe as well as a sudden dramatic drop in blood pressure. If you experience this type of allergic reaction, seek immediate emergency help. Without treatment, this condition can result in death within 15 minutes.

How Is an Allergic Reaction Diagnosed?

Allergic reactions can be diagnosed by your doctor. If you experience symptoms of an allergic reaction, your doctor will perform an exam and ask you about your health history. If your allergic reactions are severe, your doctor may ask you to keep a journal that details your symptoms and what substances appear to cause them. Your doctor may want to order tests to determine what is causing your allergy. The most common types of allergy tests are:

  • skin tests
  • challenge (elimination-type) tests
  • blood tests

A skin test involves applying a small amount of a suspected allergen to the skin and watching for a reaction. The substance may be taped to the skin (patch test), applied to a small prick in the skin (prick test), or injected just under the skin (intradermal test). A skin test is most valuable for diagnosing:

  • food allergy
  • mold, pollen, and animal dander allergy
  • penicillin allergy
  • venom allergy (such as mosquito bites or bee stings)
  • allergic contact dermatitis (a rash you get from touching a substance)

Challenge testing is useful in diagnosing food allergies. It involves removing a food from your diet for several weeks and watching for symptoms when you eat the food again.

A blood test for an allergy checks your blood for antibodies against a possible allergen. An antibody is a protein your body produces to fight harmful substances. Blood tests are used when skin testing is not helpful or cannot be done.

How Is an Allergic Reaction Treated?

If you experience an allergic reaction and you do not know what is causing it, you may need to see your doctor to determine what you are allergic to. If you have a known allergy and experience symptoms, you may not need to seek medical care if your symptoms are mild. In most cases, over-the-counter antihistamines, such as Benadryl, can be effective for controlling mild allergic reactions.

If you or someone you know experiences a severe allergic reaction, you should seek emergency medical attention. Check to see if the person is breathing, call 911, and provide cardiopulmonary resuscitation (CPR) if needed. People with known allergies often have emergency medications with them such as an EpiPen, which injects the drug epinephrine. Epinephrine opens the airways and raises blood pressure. This is called a rescue drug. If the person is unable to administer the medication, help him or her to take it. If the person is unconscious, you should:

  • lay the person flat on his or her back
  • elevate the person’s legs
  • cover the person with a blanket

This will help prevent shock.

What Is the Long-Term Outlook?

If you have a known allergy, preventing an allergic reaction will improve your outlook. You can prevent these reactions by avoiding the allergens that affect you. If you have serious allergic reactions, you should carry an EpiPen (epinephrine) to inject yourself if symptoms occur. 

Your outlook will also depend on the severity of your allergy. If you have a mild allergic reaction and seek treatment, you will have a good chance for recovery. However, symptoms may recur if you are exposed to the allergen again. If you have a severe allergic reaction, your prognosis will depend on receiving quick emergency care. Anaphylaxis can result in death. Prompt medical care is needed to improve your outcome.

How Can You Prevent an Allergic Reaction?

You may not be able to prevent an allergic reaction. But there are steps that you can take to prevent future allergic reactions. Once you identify your allergy you can:

  • avoid exposure to the allergen
  • seek medical care if you are exposed to the allergen
  • carry medications to treat anaphylaxis
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