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

See also:

• Allergic conjunctivitis
• Allergic reactions
• Allergy testing
• Allergy to mold, dander, dust
• Atopic dermatitis (eczema)
• Contact dermatitis
• Drug allergies
• Food allergies
• Hay fever (allergic rhinitis)

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.

What part of the body is contacted by the allergen plays a role in the symptoms you develop. For example, allergens that are breathed in often cause a stuffy nose, itchy nose and throat, mucus production, cough, or wheezing. A food allergen can cause nausea, vomiting, abdominal pain, cramping, diarrhea, or a severe, life-threatening reaction. Allergies to plants often cause a skin rash. Drug allergies usually involve the whole body and can lead to a variety of symptoms.

Certain medical conditions such as sinus problems, eczema, and asthma are linked to allergies.

Common allergens include pollen, mold, pet dander, and dust. Food and drug allergies are common. Allergic reactions can also be caused by insect bites, jewelry, cosmetics, and other substances.

Some people have allergy-like reactions to hot or cold temperatures, sunlight, or other environmental triggers. Sometimes, friction (rubbing or roughly stroking the skin) will cause symptoms.

Whether or not you are likely to develop allergies is often passed down through families. If both your parents have allergies, you are likely to have allergies. The chance is greater if your mother has allergies.

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 prick test. It involves placing a small amount of the suspected allergy-causing substances on the skin, and then slightly pricking 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.

Blood tests can measure the levels of specific allergy-related substances, especially one called immunoglobulin E (IgE).

A complete blood count (CBC), specifically the eosinophil white blood cell count, may also help reveal allergies.

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.

Severe allergic reactions (anaphylaxis) require treatment with a medicine called epinephrine, which can be life saving when immediately given.

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

There are several types of medications available to prevent and treat allergies. Which medicine your doctor recommends depends on the type and severity of your symptoms, your age, and overall health.

Specific illnesses that are caused by allergies (such as asthma, hay fever, and eczema) may require other treatments.

Medications that can be used to treat allergies include:

ANTIHISTAMINES

Antihistamines are available over-the-counter and by prescription. They are available in many forms, including:

• Capsules and pills
• Eye drops
• Injection
• Liquid
• Nasal spray

CORTICOSTEROIDS

Anti-inflammatory medications (corticosteroids) are available in many forms, including:

• Eye drops
• Nasal spray
• Lung inhaler

Patients with severe allergic symptoms may be prescribed corticosteroid pills or injections for short periods of time.

DECONGESTANTS

Decongestants can help relieve a stuffy nose. Decongestant nasal spray should not be used for more than several days, because they can cause a "rebound" effect and make the congestion worse. Decongestants in pill form do not cause this problem.

OTHER MEDICINES

Leukotriene inhibitors are medicines that specifically block the substances that trigger allergies. Zafirlukast (Accolate) and montelukast (Singulair) are approved for those with asthma and indoor and outdoor allergies.

ALLERGY SHOTS

Allergy shots (immunotherapy) are occasionally recommended if the allergen cannot be avoided and symptoms are hard to control. Allergy shots keep your body from over-reacting to the allergen. Regular injections of the allergen are given, with each dose slightly larger than the previous dose until a maximum dose is reached. They do not work for everybody and require frequent doctor's visits.

See: Asthma and allergy support group

Most allergies can be easily treated with medication.

Some children may outgrow an allergy. This is particularly true of food allergies. However, as a general rule, once a substance has triggered an allergic reaction, it continues to affect the person.

Allergy shots are most effective when used to treat those with hay fever symptoms and severe insect sting allergies. They do not work well for food allergies. Allergy shots may require years of treatment, but they work in about two-thirds of cases. However, they may cause uncomfortable side effects (such as hives and rash) and dangerous outcomes (such as anaphylaxis).

• Anaphylaxis (life-threatening allergic reaction)
• Breathing problems and discomfort during the allergic reaction
• Drowsiness and other side effects of medicines

Call for an appointment with your health care provider if:

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

Breast-feeding children for at least 4 months or more may help prevent atopic dermatitis cow milk allergy, and wheezing in early childhood.

However, changing a mother's diet during pregnancy or while breast-feeding does not seem to help prevent allergy-related conditions.

For most children, changing diet or special formulas does not seem to prevent these problems. If there is a family history of eczema and allergies in a parent, brother, or sister, discuss the infant feeding with your child's doctor. The timing of introduction of solid foods in general, as well as use of several specific foods, can help prevent some allergies.

There is also evidence that infants exposed to certain airborne allergens (such as dust mites and cat dander) may be less likely to develop related allergies. This is called the "hygiene hypothesis" and sprang from observations that infants on farms tend to have fewer allergies than those who grow up in environments that are more sterile.

Once allergies have developed, treating the allergies and carefully avoiding those things that cause reactions can prevent allergies in the future.

Wallace DV, Dykewicz MS, Bernstein DI, Blessing-Moore J, Cox L, Khan DA, et al. The diagnosis and management of rhinitis: an updated practice parameter. J Allergy Clin Immunol. 2008 Aug:122(2).

Kurowski K, Boxer RW. Food allergies: detection and management. American Family Physician. 2008 June:77(12).

Bielory L, Friedlaender MH. Allergic conjunctivitis. Immunol Allergy Clin North Am. 2008 Feb;28(1):43-58, vi.