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. Individuals with allergies fall into two categories: those who can have life-threatening reactions when exposed (called anaphylaxis), and those who have symptoms that cause discomfort without being life-threatening

In a person who develops this misdirected reaction, 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 usually 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; these antibodies bind to cells that contain histamine. Histamine (and other chemicals) are released by these cells when the antibodies are stimulated by allergens. This causes itching, swelling of affected tissues, mucus production, muscle spasms, hives, 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, cough, or wheezing. Ingestion of a food allergen, in addition to the risk of causing a severe, life-threatening reaction, can cause nausea, vomiting, abdominal pain, cramping, or diarrhea. 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 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 relatively common. Both hereditary and environmental factors have been found to contribute to a predisposition to developing allergies.

• runny nose (rhinitis)
• tearing, increased
• itching of the nose, mouth, eyes, throat, skin, or any area
• wheezing
• coughing
• difficulty breathing
• hives or skin wheals
• skin rashes
• stomach cramps
• vomiting
• diarrhea
• headache
• chemosis and red, itchy eyes

• Antibody/immunoglobulin (particularly IgE) levels that are elevated indicate a 'primed' immune system.
• The CBC may reveal an increase in eosinophils.
• 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. Skin testing may even be an option for young children and infants, depending on the circumstances.
  • The blood test or RAST (radioallergosorbent) test 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.
  • 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.)
  • Other testing to determine the specific causative 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. This is often employed when foods or medications are being evaluated as possible allergens.
  • Tests for reaction to physical stimuli may include application of the stimuli (heat, cold, and so on) and observation for an allergic response.

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

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 (Benedryl).
• 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), cetirizine (Zyrtec), and loratadine (Claritin).
• For people with symptoms not relieved by antihistamines alone, nasal corticosteroid sprays are very effective and safe. 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) is occasionally recommended if the allergen cannot be avoided and symptoms are hard to control. It includes regular injections of the allergen, given in increasing doses (each dose is slightly larger than the previous dose) that may help the body adjust to the antigen.

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

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

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.

Desensitization may cause uncomfortable side effects (such as hives and rash) and may have dangerous outcomes (such as anaphylaxis). It is most effective in allergens that cause hayfever symptoms and individuals with severe insect sting allergies, but not for food allergens. 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.