Causes

Allergies are pretty common. Both genes and environmental factors play a role.

The immune system normally protects the body against harmful substances, such as bacteria and viruses. It also reacts to foreign substances called allergens, which 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. which fight off the allergen. This causes allergy symptoms.

Watch this video about:Allergies

Common allergens include:

• Drugs
• Dust
• Food
• Insect bites
• Mold
• Pet dander
• Pollen

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.

A specific allergy is not usually passed down through families (inherited). However, if both your parents have allergies, you are likely to have allergies. The chance is greater if your mother has allergies.

Allergies may make certain medical conditions such as sinus problems, eczema, and asthma worse.

Symptoms

Allergy symptoms may include:

• Breathing problems (coughing, shortness of breath)
• 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 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, or 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, diarrhea, or a severe, life-threatening reaction
• 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 health care provider will perform a physical exam and ask questions, such as when the allergy occurs.

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 and ampicillin) 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. 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.

Other types of skin tests include patch testing and intradermal testing. For more information, see: Allergy testing

• Blood tests can measure the levels of allergy-related substances, especially one called immunoglobulin E (IgE).
• A complete blood count (CBC) called the eosinophil white blood cell count may also help diagnose 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 applying 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.

Treatment

Severe allergic reactions (anaphylaxis) need to be treated with a medicine called epinephrine, which can be life saving 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.

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

Illnesses that are caused by allergies (such as asthma, hay fever, and eczema) may need 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:

• Creams and ointment for the skin
• 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. Do not use decongestant nasal spray 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. People with high blood pressure, heart problems, or prostate enlargement should use decongestants with caution.

OTHER MEDICINES

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

ALLERGY SHOTS

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. You will get regular injections of the allergen. Each dose is slightly larger than the last dose until a maximum dose is reached. These shots do not work for everybody and you will have to visit the doctor often.

Support Groups

See: Asthma and allergy support group

Outlook (Prognosis)

Most allergies can be easily treated with medication.

Some children may outgrow an allergy, especially food allergies. However, once a substance has triggered an allergic reaction, it usually continues to affect the person.

Allergy shots are most effective when used to treat people with hay fever symptoms and severe insect sting allergies. They are not used to treat food allergies because of the danger of a severe reaction.

Allergy shots may need years of treatment, but they work in most cases. However, they may cause uncomfortable side effects (such as hives and rash) and dangerous outcomes (such as anaphylaxis).

Possible Complications

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

When to Contact a Medical Professional

Call for an appointment with your health care provider if:

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

Prevention

Breastfeeding children for at least 4 months or more may help prevent a cow's milk allergy and wheezing in early childhood.

However, changing a mother's diet during pregnancy or while breastfeeding does not seem to help prevent allergies.

For most children, changing the diet or using special formulas does not seem to prevent allergies. If a parent, brother, sister, or other family member has a history of eczema and allergies, discuss feeding with your child's doctor. When you introduce solid foods and what foods you give your baby can help prevent some allergies.

There is also evidence that infants who are exposed to certain allergens in the air (such as dust mites and cat dander) may be less likely to develop allergies. This is called the "hygiene hypothesis." It came from the observation that infants on farms tend to have fewer allergies than those who grow up in more sterile environments. However, older children do not seem to benefit.

Once allergies have developed, treating the allergies and carefully avoiding allergy triggers can prevent reactions in the future.

References

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.

Sicherer S, Sampson HA. Journal of Allergy and Clinical Immunology 2010 Feb 125 (2 suppl2) S116-25.

Updated: 4/1/2012

Reviewed by: Stuart I. Henochowicz, MD, FACP, Associate Clinical Professor of Medicine, Division of Allergy, Immunology, and Rheumatology, Georgetown University Medical School. David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.