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

See also:

• hay fever
• allergy to mold, dander, dust
• food allergies
• drug allergies
• allergy testing
• allergic reactions

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. In contrast, an allergic reaction is when the immune system reacts to substances (allergens) that are generally harmless and in most people do not cause an immune response.

In a person with allergies, the first exposure to the allergen triggers the immune system to recognize the substance. Any exposure after that will usually result in symptoms.

When an allergen enters the body of a person with a sensitized immune system, histamine and other chemicals are released by certain cells. This causes itching, swelling, mucus production, muscle spasms, hives, rashes, and other symptoms.

Symptoms vary in severity from person to person. Most people have symptoms that cause discomfort without being life-threatening. A few people have life-threatening reactions (called anaphylaxis).

The part of the body contacted by the allergen will, in part, affect the symptoms. For example, allergens that are inhaled often cause nasal congestion, 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 skin rash. Drug allergies usually involve the whole body.

Some disorders may be associated with allergies. These include eczema and asthma, among others.

Common allergens include those that contact the skin, breathing passages, or the surface of the eye (such as pollen; see also allergy to mold, dander, dust). Food allergies and drug allergies are common. Allergic reactions can be caused by insect bites, jewelry, cosmetics, and almost any substance that contacts the body.

Some people have allergic-type reactions to hot or cold temperatures, sunlight, or other physical stimuli. In some persons, friction (rubbing or vigorously stroking the skin) will cause symptoms. (See also atopic dermatitis and contact dermatitis.)

Allergies are relatively common. Both hereditary and environmental factors have been found to play a role.

• runny nose
• tearing eyes, burning or itching eyes
• red eyes, conjunctivitis
• swollen eyes
• itching nose, mouth, throat, skin, or any other area
• wheezing
• coughing
• difficulty breathing
• hives (skin wheals)
• skin rashes
• stomach cramps
• vomiting
• diarrhea
• headache

The history of your symptoms is important in diagnosing all allergies, including whether the symptoms vary according to time of day, season, exposure to pets and other potential allergens, and diet changes. Severe reactions often develop very quickly after exposure, such as eating culprit nuts or getting stung.

Allergy testing may be required to determine if your symptoms are an actual allergy or caused by other problems. For example, eating contaminated food (food poisoning) may cause symptoms that resemble food allergies. Some medications (such as aspirin, ampicillin, and others) can produce non-allergic reactions, including rashes, that resemble drug allergies but are not true allergies.

Tests that may reveal the specific allergens include:

• Skin testing -- the most common method of allergy testing. This may include intradermal, scratch, patch, or other tests. Skin testing may even be an option for young children and infants, depending on the circumstances.
• Blood test -- also called RAST (radioallergosorbent), this measures the levels of allergy antibody, IgE, produced when your blood is mixed with a series of allergens in a laboratory. If you are allergic to a substance, the IgE levels may increase in the blood sample. The blood test may be used if you have existing skin problems like eczema, if you're on medications that are long-acting or you cannot stop taking, if you have a history of anaphylaxis, or if you prefer not to have a skin test.
• "Use" or "elimination" tests -- suspected items are eliminated and/or introduced while the person is observed for response to the substance. This is often used to check for food or medication allergies.
• Eyelid -- Occasionally, the suspected allergen is dissolved and dropped onto the lining of the lower eyelid (conjunctiva) as a means of testing for allergies. (This test should only be done by a physician, never the patient, since it can be harmful if done improperly.)
• Reaction to physical stimuli -- application of heat, cold, or other stimulation, and then look for an allergic response.

Other tests that may reveal allergies include:

• Antibody/immunoglobulin (particularly IgE) levels -- when these are elevated, it indicates a "primed" immune system.
• CBC -- may reveal an increase in eosinophils.
• Complement levels -- may be abnormal.

The goal is to reduce the symptoms caused by inflammation of the affected tissues.

Of course, the best "treatment" is to avoid what causes your allergies in the first place. It may be impossible to completely avoid everything you are allergic to, but you can often take steps to reduce your exposure. This is especially important for food and drug allergies.

Medications that can be used to treat allergies include the following:

• Short-acting antihistamines, which are generally non-prescription, often relieve mild to moderate symptoms but can cause drowsiness. In addition, these antihistamines can blunt learning in children (even in the absence of drowsiness). An example is diphenhydramine. One formerly prescription medication, loratadine (Claritin), is now available over the counter. It does NOT tend to cause drowsiness or affect learning in children.
• Longer-acting antihistamines cause less drowsiness and can be equally effective, and usually do not interfere with learning. These medications, which require a prescription, include fexofenadine (Allegra) and cetirizine (Zyrtec).
• Nasal corticosteroid sprays are very effective and safe for people with symptoms not relieved by antihistamines alone. These prescription medications include fluticasone (Flonase), mometasone (Nasonex), and triamcinolone (Nasacort AQ).
• Decongestants may also be helpful in reducing symptoms such as nasal congestion. Nasal spray decongestants 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 effect.
• Cromolyn sodium is available as a nasal spray (Nasalcrom) for treating hay fever. Eye drop versions of cromolyn sodium and antihistamines are available for itchy, bloodshot eyes.

The most appropriate medication depends on the type and severity of symptoms. Specific illnesses that are caused by allergies (such as asthma, hay fever, and eczema) may require other treatments.

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

Severe reactions (anaphylaxis) require epinephrine, which can be life saving when administered soon after exposure by patients themselves.

Most allergies can be easily treated with medication. However, medication only helps relieve the symptoms from that exposure, they do not prevent future allergic reactions.

Some children may outgrow an allergy as the immune system becomes less sensitive to the allergen. This is particularly true of food allergies. However, as a general rule, once substances have provoked allergic reactions, they continue to affect the person.

Allergy shots may cause uncomfortable side effects (such as hives and rash) and may have dangerous outcomes (such as anaphylaxis). Allergy shots are most effective in allergens that cause hayfever symptoms and individuals with severe insect sting allergies, but not food allergies. While it may require years of treatment, it is effective in about two-thirds of cases.

• discomfort during the allergic reaction
• disruption of lifestyle
• drowsiness and other side effects of antihistamines
• side effects of other medications (see the specific medication)
• anaphylaxis (life-threatening allergic reaction)

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.