Causes, incidence, and risk factors:
Allergy is caused by hypersensitivity of the immune system leading to a misdirected immune response. The immune system normally protects the body against harmful substances such as bacteria, viruses, and toxins. Allergy occurs when the immune system reacts to substances (allergens) that are generally harmless and in most people do not cause an immune response.

The first exposure to the allergen causes a mild immune response that sensitizes the immune system to the substance (triggers the immune system to recognize the substance). The second and subsequent exposure to the allergen results in symptoms. The type of symptom that develops depends on the specific allergen, the part of the body where exposure occurs, and the way the immune system reacts to the allergen.

When an allergen enters the body of a person with a sensitized immune system, it triggers antibody production. Histamine and other chemicals are released by body tissues as part of the immune response. This causes itching, swelling of affected tissues, mucus production, muscle spasms, hives and rashes, and other symptoms. Symptoms vary in severity from person to person.

The part of the body contacted by the allergen will, in part, also affect symptoms. For example, allergens that are inhaled often cause nasal congestion, itchy nose/throat, mucus production, coughing, wheezing, or similar symptoms. Food allergies often include abdominal pain, cramping, or similar symptoms, although the whole body may be affected when the food is absorbed. Allergies to plants often cause skin rash. Drug allergies usually involve the whole body.

Many disorders are associated with, triggered, or worsened by allergies. These include hay fever, eczema, asthma, and many others.

Common allergens include environmental agents 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 (such as rubbing or vigorously stroking the skin) will cause symptoms. The mechanism that causes this is not well understood, but it is possible that minute changes in the chemistry of the skin may occur in response to physical stimuli and some component of this chemical change triggers the allergy. (See also atopic dermatitis and contact dermatitis.).

Allergies are common. Heredity, environmental conditions, number and type of exposures, emotional factors (stress and emotional upset can increase the sensitivity of the immune system), and many other factors can indicate a predisposition to allergies.
 

Prevention:
There is no known way to prevent allergies. Symptoms may be prevented by avoiding known allergens.

Symptoms:

• runny nose (rhinitis)
• tearing, increased
• itching of the nose, mouth, eyes, throat, skin, or any area
• wheezing
• coughing
• difficulty breathing
• skin redness
• hives
• skin rashes
• stomach cramps
• vomiting
• diarrhea
• bloated feeling of the stomach
• headache
• hearing loss
• ear discharges/bleeding
• chemosis

Note: Allergies vary according to the type of antigen and the part of the body in which the allergic reaction occurs.

Signs and tests:
History is important in diagnosing allergies, including whether the symptoms vary according to time of day or the season and possible exposures that involve pets, diet changes, or other sources of allergens.

Testing may be required to determine if symptoms are an actual allergy or caused by other problems. For example, contaminated food ("food poisoning") may 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.
 

• Antibody/immunoglobulin (particularly IgE) levels that are elevated indicate allergic reaction.
• Complement levels may be abnormal.
• Testing may reveal the specific allergen(s).
  • Skin testing is the most common method of allergy testing. This may include intradermal, scratch, patch, or other tests.
  • Occasionally, the suspected allergen is dissolved and dropped onto the lining of the lower eyelid (conjunctiva) as a means of testing for allergies.
  • Other testing to determine the specific allergen may include various types of "use" or "elimination" tests where suspected items are eliminated and/or introduced while the person is observed for response to the substance.
  • Tests for reaction to physical stimuli may include application of the stimuli (heat, cold, and so on) and observation for an allergic response.

This disease may also alter the results of the following tests:

• WBC count
• CBC
• immunoelectrophoresis - serum

Treatment:
Treatment varies with the severity and type of symptom. Short-term goals include relieving immediate symptoms. Long-term goals include avoiding future allergic reactions.

Corticosteroids such as dexamethasone or prednisone reduce the immune response and may be prescribed to reduce symptoms. Epinephrine is used to reduce swelling of the airways and other life-threatening symptoms. Antihistamines such as diphenhydramine often provide good relief of mild to moderate symptoms.

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

Avoidance of the allergen is the best long-term treatment, particularly with allergic reaction to foods or medications

Desensitization (immunotherapy, "allergy shots") is occasionally recommended if the allergen cannot be avoided. It includes regular injections of the allergen, given in increasing doses that may "acclimatize" the body to the antigen.

Support groups:
The stress of illness can often be helped by joining a support group where members share common experiences and problems. See asthma and allergy - support group.

Expectations (prognosis):
Most allergies are readily treated. However, treatment only affects that exposure, so subsequent exposures cause another allergic reaction.

Rarely, people may "outgrow" an allergy as the immune system becomes less sensitive to the allergen. However, as a general rule, once a substance has provoked an allergic reaction, it continues to affect the person, and may cause an increasingly severe response with repeated exposures.

Desensitization may cause uncomfortable side effects (such as hives and rash) and may have dangerous outcomes (such as anaphylaxis). It may require years of treatment and is effective in about two-thirds of cases.

Complications:

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