Allergies are abnormal reactions of the immune system that occur in response to otherwise harmless substances.
Allergies are among the most common medical disorders. It is estimated that 60 million Americans, or more than one in every five people, suffer from some form of allergy, with similar proportions throughout much of the rest of the world. Allergy is the single largest reason for school absence and is a major source of lost productivity in the workplace.
An allergy is a type of immune reaction. Normally, the immune system responds to foreign bodies, like pollen or bacteria, by producing specific proteins called antibodies that are capable of binding to identifying molecules (antigens) on the foreign body. This reaction between antibody and antigen sets off a series of reactions designed to protect the body from infection. Harmless, everyday substances trigger this same series of reactions. This is the condition known as allergy, and the offending substance is called an allergen.
Allergens enter the body through four main routes: the airways, the skin, the gastrointestinal tract, and the circulatory system.
- Airborne allergens cause the sneezing, runny nose, and itchy, bloodshot eyes of hay fever (allergic rhinitis). Airborne allergens can also affect the lining of the lungs, causing asthma, or the conjunctiva of the eyes, causing conjunctivitis (pink eye).
- Allergens in food can cause itching and swelling of the lips and throat, cramps, and diarrhea. When absorbed into the bloodstream, they may cause hives or more severe reactions involving recurrent, non-inflammatory swelling of the skin, mucous membranes, organs, and brain (angioedema). Some food allergens may cause anaphylaxis, a potentially life-threatening condition marked by tissue swelling, airway constriction, and drop in blood pressure.
- In contact with the skin, allergens can cause reddening, itching, and blistering, called contact dermatitis. Skin reactions can also occur from allergens introduced through the airways or gastrointestinal tract. This type of reaction is known as atopic dermatitis.
- Injection of allergens, from insect bites and stings or drug administration, can introduce allergens directly into the circulation, where they may cause system-wide responses (including anaphylaxis), as well as the local responses like swelling and irritation at the injection site.
People with allergies are not equally sensitive to all allergens. Allergies may get worse over time. For example, childhood ragweed allergy may progress to year-round dust and pollen allergy. On the other hand, a person may lose allergic sensitivity. Infant or childhood atopic dermatitis disappears in almost all people. More commonly, what seems to be loss of sensitivity is instead a reduced exposure to allergens or an increased tolerance for the same level of symptoms.
Causes & symptoms
Immunologists separate allergic reactions into two main types: immediate hypersensitivity reactions, which are mainly mast cell-mediated and occur within minutes of contact with allergen, and delayed hypersensitivity reactions, mediated by T cells (a type of white blood cells) and occurring hours to days after exposure.
In the upper airways and eyes, immediate hypersensitivity reactions cause the runny nose and itchy, bloodshot eyes typical of allergic rhinitis. In the gastrointestinal tract, these reactions lead to swelling and irritation of the intestinal lining, which causes the cramping and diarrhea typical of food allergy. Allergens that enter the circulation may cause hives, angioedema, anaphylaxis, or atopic dermatitis.
Allergens on the skin usually cause delayed hypersensitivity reaction. Roving T cells contact the allergen, setting in motion a more prolonged immune response. This type of allergic response may develop over several days following contact with the allergen, and symptoms may persist for a week or more.
THE ROLE OF INHERITANCE. While allergy to specific allergens is not inherited, the likelihood of developing some type of allergy seems to be, at least for many people. If neither parent has allergies, the chances of a child developing allergy is approximately 10–20%; if one parent has allergies, it is 30–50%; and if both have allergies, it is 40–75%.
COMMON ALLERGENS. The most common airborne allergens are the following:
- plant pollens
- animal fur and dander
- body parts from house mites (microscopic creatures found in all houses)
- house dust
- mold spores
- cigarette smoke
Common food allergens include the following:
- nuts, especially peanuts, walnuts, and Brazil nuts
- fish, mollusks, and shellfish
- food additives and preservatives
Common causes of contact dermatitis include the following:
- poison ivy, oak, and sumac
- nickel or nickel alloys
Insects and other arthropods whose bites or stings typically cause allergy include the following:
- bees, wasps, and hornets
Symptoms depend on the specific type of allergic reaction. Allergic rhinitis is characterized by an itchy, runny nose often with a scratchy or irritated throat due to postnasal drip. Inflammation of the thin membrane covering the eye (allergic conjunctivitis) causes redness, irritation and increased tearing in the eyes. Asthma causes wheezing, coughing, and shortness of breath. Symptoms of food allergies depend on the tissues most sensitive to the allergen and whether it is spread systemically by the circulatory system. Gastrointestinal symptoms may include swelling and tingling in the lips, tongue, palate or throat; nausea; cramping; diarrhea; and gas. Contact dermatitis is marked by reddened, itchy, weepy skin blisters.
Whole body or systemic reactions may occur from any type of allergen, but are more common following ingestion or injection of an allergen. Skin reactions include the raised, reddened, and itchy patches called hives. A deeper and more extensive skin reaction, involving more extensive fluid collection, is called angioedema. Anaphylaxis, another reaction, is marked by difficulty breathing, blood pressure drop, widespread tissue swelling, heart rhythm abnormalities, lightheadedness, and in some cases, loss of consciousness.
Allergies can often be diagnosed by a careful medical history, matching the onset of symptoms to the exposure to possible allergens. Allergy tests can be used to identify potential allergens. These tests usually begin with prick tests or patch tests, which expose the skin to small amounts of allergen to observe the response. Reaction will occur on the skin even if the allergen is normally encountered in food or in the airways. RAST testing is a blood test that measures the level of reactive antibodies in the blood. Provocation tests, most commonly done with airborne allergens, present the allergen directly through the route normally involved. Food allergen provocation tests require abstinence from the suspect allergen for two weeks or more, followed by ingestion of a measured amount. Provocation tests are not used if anaphylaxis is a concern due to the patient's medical history.
The following can help to relieve the symptoms of airborne allergies:
- Stinging nettle (Urtica dioica) has antihistamine and anti-inflammatory properties. The common dose is 300 mg four times daily.
- Grape (Vitis vinifera) seed extract has antihistamine and anti-inflammatory properties. The usual dose is 50 mg three times daily.
- Ephedra (Ephedra sinicia), also called ma huang, has anti-inflammatory activity and is proven effective in treating allergies. Taken as a tea thrice daily or 12.5–25 mg in capsule form. Ephedra should not be used for prolonged periods of time, as it can raise blood pressure, cause rapid heartbeat, and interfere with adrenal gland function.
- Licorice (Glycyrrhiza glabra) has cortisone-like activity which is anti-inflammatory, stimulates the adrenals, and relieves allergy symptoms. Can be taken as a tea or 100–300 mg in capsule form. Long term use can result in sodium retention or potassium loss.
- Chinese skullcap (Scutellaria baicalensis) has bronchodilator activity, is an anti-inflammatory, and prevents allergic reactions. It is taken in combination with other herbs.
- Ginkgo (Ginkgo biloba) seeds are used in Chinese medicine for wheezing and coughing.
- Echinacea (Echinacea species) may have antiinflammatory activity and boost the immune system.
- Khellin (Ammi visnaga) has bronchodilator activity.
- Cramp (Viburnum opulus) bark has bronchodilator activity.
- Traditional Chinese medicine treats allergic rhinitis with various species. Patent combination medicines are: Bu Zhong Yi Qi Wan (Tonify the Middle and Augment the Qi) and Yu Ping Feng San (Jade Windscreen) are used for preventing allergies, and Bi Yan Pian (Rhinitis Infusion) is often prescribed for syptoms affecting the nose.
- The homeopathic remedies Rhus toxicodendron, Apis mellifica, and Nux vomica have decongestant activities. They are taken internally.
- Vitamin C has antihistamine and decongestant activities.
- Vitamins A and E are antioxidants and help to promote normal functioning of the immune system.
- Coenzyme Q10 may help to promote normal functioning of the immune system.
- Zinc may boost the immune system.
- N-acetylcysteine may have decongestant activity.
- Acupuncture has been shown to be as effective as antihistamine drugs in treating allergic rhinitis. It is also used to help prevent allergic reactions by strengthening the immune system.
A variety of herbal remedies, either applied topically or taken internally, can assist in the treatment of contact dermatitis. A poultice made of jewelweed (Impatiens species) or chickweed (Stellaria media) can soothe the skin. A cream or wash containing calendula (Calendula officinalis), a natural antiseptic and anti-inflammatory agent, can help heal the rash. Chinese herbal remedies have been effective in treating atopic dermatitis. The following are homeopathic remedies to be taken internally:
- Apis (Apis mellifica) for hives that feel better with cold application and bee stings
- Poison ivy (Rhus toxicodendron) for hives that feel better with hot applications and for poison ivy, oak, or sumac rashes
- Stinging nettle (Urtica urens) for hives
- Marsh tea (Ledum) for itching insect bites
- Croton (Croton tiglium) oil for poison ivy, oak, or sumac rashes
Food allergy may be managed by oral desensitization. Children with allergy to milk, eggs, fish, or apples who followed an oral desensitization procedure developed resistance to the allergenic food. Oral desensitization exposes the patient to allergens in controlled, but increasing, doses. Control subjects, who had avoided the allergenic food during the study, were still sensitive.
A large number of prescription and over-the-counter drugs are available for treatment of immediate hypersensitivity reactions. Most of these drugs work by decreasing the ability of histamine to provoke symptoms. Other drugs counteract the effects of histamine by stimulating other systems or reducing immune responses in general.
ANTIHISTAMINES. Antihistamines block the histamine receptors on nasal tissue, decreasing the effect of histamine released by mast cells. They may be used after symptoms appear, though they seem to prove more effective when used preventively. A wide variety of antihistamines are available.
DECONGESTANTS. Decongestants constrict blood vessels to counteract the effects of histamine. Nasal sprays and oral systemic preparations are available. Decongestants are stimulants and may cause increased heart rate and blood pressure, headaches, and agitation. Use of nasal sprays for longer than several days can cause loss of effectiveness and rebound congestion, in which nasal passages become more severely swollen than before treatment.
TOPICAL CORTICOSTEROIDS. Topical corticosteroids reduce mucous membrane and skin inflammation and are available by prescription. Allergies tend to become worse as the season progresses and topical corticosteroids are especially effective at reducing this seasonal sensitization. As a result, they are best started before allergy season begins. A 2001 study revealed that steroid nasal sprays work better for seasonal allergies on an as-needed basis than do antihistamines. Side effects are usually mild, but may include headaches, nosebleeds, and unpleasant taste sensations.
MAST CELL STABILIZERS. Cromolyn sodium prevents the release of mast cell granules, thereby preventing the release of histamine and other chemicals contained in them. Cromolyn sodium is available as a nasal spray and aerosol (a suspension of particles in gas).
BRONCHODILATORS. Because allergic reactions involving the lungs cause the airways or bronchial tubes to narrow, bronchodilators, which cause the smooth muscle lining the airways to open, can be very effective. Bronchodilators include adrenaline, albuterol, and theophylline. Other drugs, including steroids, are used to prevent and control asthma attacks.
Immunotherapy, also known as desensitization or allergy shots, alters the balance of antibody types in the body. Injections involve gradually increasing amounts of allergen, over several weeks or months, with periodic boosters. Full benefits may take up to several years to achieve and are not seen at all in about one in five patients. Individuals receiving all shots will be monitored closely following each shot because of the small risk of anaphylaxis, a condition that can result in difficulty breathing and a sharp drop in blood pressure.
Possible future treatments
In late 2001, a reports stated that a monoclonal antibody called omalizumab might be effective in treating seasonal allergies and preventing related asthma. By blocking immunoglobulin E (IgE), an antibody that is found in excessive amounts in those with hay fever, the drug treats hay fever and helps prevent related asthma. Trials on the drug continue, and other immune-based medicines will likely accompany its ultimate release.
Treatment of contact dermatitis
Calamine lotion applied to affected skin can reduce irritation somewhat. Topical corticosteroid creams are more effective, though overuse may lead to dry and scaly skin.
Treatment of anaphylaxis
The emergency condition of anaphylaxis is treated with injection of adrenaline, also known as epinephrine. People who are prone to anaphylaxis because of food or insect allergies often carry an "Epi-pen" containing adrenaline in a hypodermic needle. Prompt injection can prevent a more serious reaction from developing.
Allergies can improve over time, although they often worsen. While anaphylaxis and severe asthma are life-threatening, other allergic reactions are not. Learning to recognize and avoid allergy-provoking situations allows most people with allergies to lead normal lives.
By determining which allergens are causing the reactions, most people can learn to avoid allergic reactions from food, drugs, and contact allergens. Airborne allergens are more difficult to avoid, although keeping dust and animal dander from collecting in the house may limit exposure. Vitamin C may prevent allergy symptoms. Cromolyn sodium can be used for allergy prevention.
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