Photo of woman suffering from seasonal allergies

Condition or Illness – What is an Allergy?

An allergy is an immune response or reaction to substances that are usually not harmful.

Causes of Allergies

Both genes and the environment play a role in allergies, which are extremely common. The immune system protects the body against harmful substances, such as bacteria and viruses, and reacts to foreign substances (allergens). In most people allergens are harmless, but in a person with allergies, the immune response is oversensitive. When it recognizes an allergen, the immune system launches a response.

Chemicals, called histamines are released. These chemicals cause allergy symptoms.

Common allergens include:

  • Drugs
  • Dust
  • Food
  • Insect venom-bee stings, mosquito bites
  • Mold
  • Pet and other animal dander
  • Pollen

Signs and Symptoms

The part of the body the allergen touches affects what symptoms you develop. For example:

  • Allergens that you breathe in often cause a stuffy nose, itchy nose and throat, mucus production, cough, and wheezing.
  • Allergens that touch the eyes may cause itchy, watery, red, swollen eyes.
  • Eating something you are allergic to can cause nausea, vomiting, abdominal pain, cramping, or diarrhea.
  • Allergens that touch the skin can cause a skin rash, hives, itching, blisters, or skin peeling.
  • Drug allergies usually involve the whole body and can lead to a variety of symptoms.

Exams and Tests

The first step in examining someone for allergies is to determine when the allergy occurs. Your physician may ask questions regarding the symptoms you're experiencing, what you think brought upon those symptoms and what alleviates those symptoms.

Allergy testing may be needed to find out whether the symptoms are an actual allergy or are caused by other problems. For example, eating contaminated food (food poisoning) may cause symptoms similar to food allergies. Some medications (such as aspirin) can produce non-allergic reactions, including rashes. A runny nose or cough may actually be due to an infection.

Skin testing is the most common method of allergy testing.

Other types of skin tests include patch testing and intradermal testing.

Blood tests can measure the levels of allergy-related substances.

In some cases, the doctor may tell you to avoid certain items to see if you get better, or to use suspected items to see if you feel worse. This is called "use or elimination testing." This is often used to check for food or medication allergies.

The doctor may also check your reaction to physical triggers by applying heat, cold, or other stimulation to your body and watching for an allergic response.


Severe allergic reactions (anaphylaxis) need to be treated with a medicine called epinephrine, which can be lifesaving when given right away. If you use epinephrine, call 911 and go straight to the hospital.

The best way to reduce symptoms is to avoid what causes your allergies. This is especially important for food and drug allergies.

The medicine your doctor recommends depends on the type and severity of your symptoms, your age, and overall health. These include antihistamines, corticosteroids and decongestants to relieve symptoms.

Allergy shots (immunotherapy) are sometimes recommended if you cannot avoid the allergen and your symptoms are hard to control. Allergy shots keep your body from over-reacting to the allergen.

When to contact a medical professional

Call 911 immediately if anaphylaxis or other life threatening reactions occur. Call your health care provider if:

  • Severe symptoms of allergy occur
  • Treatment for allergies no longer works


Research suggests the following may reduce the risk for developing allergies:

  • Breastfeeding
  • There is also evidence that being exposed to certain allergens (such as dust mites and cat dander) in the first year of life may prevent some allergies. This is called the "hygiene hypothesis."
  • Once allergies have developed, treating the allergies and carefully avoiding allergy triggers can prevent reactions in the future.

You should always consult your health care professional before beginning or stopping any medication regimen as adverse reaction may occur.

We would like to thank the National Library of Health's MedLine plus for some of the information contained in this article.

Further Information

For more information regarding this topic and others please visit the National Library of Health's MedLinePlus website.