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

See also:

• Allergic reactions
• Allergy testing
• Allergy to mold, dander, dust
• Drug allergies
• Food allergies
• Hay fever

Allergies are relatively common. Both genetics and environmental factors play a role.

Allergy is caused by an oversensitive immune system, which leads to a misdirected immune response. The immune system normally protects the body against harmful substances, such as bacteria and viruses. It reacts to substances (allergens) that are generally harmless and in most people do not cause a problem.

But in a person with allergies, the immune response is oversensitive. When it recognizes an allergen, it releases chemicals, such as histamines. This causes itching, swelling, mucus production, muscle spasms, hives, rashes, and other symptoms, which vary from person to person.

Allergy symptoms vary, but may include:

• Breathing problems
• Burning, tearing, or itchy eyes
• Conjunctivitis (red, swollen eyes)
• Coughing
• Diarrhea
• Headache
• Hives
• Itching of the nose, mouth, throat, skin, or any other area
• Runny nose
• Skin rashes
• Stomach cramps
• Vomiting
• Wheezing

The health care provider will perform a physical exam and ask questions such as when the allergy occurs.

Allergy testing may be needed to determine if the symptoms are an actual allergy or caused by other problems. For example, eating contaminated food (food poisoning) may cause symptoms similar to food allergies. Some medications (such as aspirin and ampicillin) can produce non-allergic reactions, including rashes.

Skin testing is the most common method of allergy testing. One type of skin testing is the scratch test. It involves placing a small amount of the suspected allergy-causing substances on the skin, and then slightly scratching the area so the substance moves under the skin. The skin is closely watched for signs of a reaction, which include swelling and redness. Skin testing may be an option for some young children and infants.

A blood test called RAST (radioallergosorbent) measures the levels of specific allergy-related substances. The blood test may be used if you cannot have or do not want a skin test.

Other blood tests that may help reveal allergies include:

• Antibody/immunoglobulin (particularly IgE) levels
• CBC
• Complement levels

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 apply heat, cold, or other stimulation to your body and watching for an allergic response.

Sometimes, a suspected allergen is dissolved and dropped into the lower eyelid to check for an allergic reaction. This should only be done by a health care provider.