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{{dablink|This article deals specifically with [[IgE]]-mediated hypersensitivity. For other types of allergic or hypersensitive reactions see the main article: [[Hypersensitivity]]}}
'''This is a background article'''. See [[Psychological factors in allergic reactions]]
The word "allergy" was coined by the Viennese pediatrician [[Clemens von Pirquet]] in 1906.<ref>Clemens Peter Pirquet von Cesenatico {{WhoNamedIt|Doctor|2382}}</ref> Pirquet noted that some of his patients were hypersensitive to normally innocuous entities such as dust, pollen, or certain foods. He called this phenomenon "allergy", from the Greek words ''allos'' meaning "other" and ''ergon'' meaning "work".<ref>[ Allergy Glossary, Health on the Net Foundation]</ref>
Historically, all forms of hypersensitivity were classified as allergies, and all thought to be caused by an improper activation of the [[immunoglobulin|antibody]] class called [[Immunoglobulin E]] (IgE){{Fact|date=March 2007}}. Later, it became clear that several different disease mechanisms were implicated, with the common link between these varying hypersensitivities being a disordered activation of the immune system in one way or another. A new classification scheme was designed by P. Gell and R. Coombs{{Fact|date=March 2007}} to reflect what were then rebaptized [[hypersensitivities|hypersensitivity reactions]]. The word "allergy" was then restricted to type I hypersensitivities, which are caused by the classical IgE mechanism.
Type I hypersensitivity is characterised by excessive activation of [[mast cell]]s and [[basophil granulocyte|basophils]] by IgE, resulting in a systemic [[Inflammation|inflammatory response]] that can result in symptoms as benign as a [[rhinorrhea|runny nose]], to life-threatening [[anaphylaxis|anaphylactic shock]] and death.
Knowing any allergies that a patient may have is important in a clinical setting. Full allergy history is taken down when obtaining a medical history of a patient. This ensures that no contradictory treatments be prescribed to the danger of the patient.{{Fact|date=March 2007}}
Allergy is a very common disorder and more than 50 million Americans suffer from allergic diseases. Allergies are the sixth leading cause of chronic disease in the United States, costing the health care system $18 billion annually. In fact, 60% of people with allergies are said to develop asthma as well.<ref> O'Brien, T. "Your Health Questions Answered!" (July 2004) ''[[Fitness Magazine]]'', p 80.</ref><!--Is this IgE allergies??-->
==Signs and symptoms==
Allergy is a local or systemic [[inflammation|inflammatory]] response to allergens. Local symptoms are:
* [[Nose]]: swelling of the nasal mucosa ([[allergic rhinitis]])
* [[Eye]]s: redness and [[itch]]ing of the [[conjunctiva]] ([[allergic conjunctivitis]])
* [[Airway]]s: [[bronchoconstriction]], wheezing and dyspnoea, sometimes outright attacks of [[asthma]], in severe cases the airway constricts due to swelling known as [[anaphylaxis]].
* [[Ear]]s: feeling of fullness, possibly pain, and impaired hearing due to the lack of [[eustachian tube]] drainage.
* [[Skin]]: various [[rash]]es, such as [[eczema]], [[hives]] ([[urticaria]]) and [[contact dermatitis]].
* [[Head]]: while not as common, headaches are seen in some with environmental or chemical allergies.
Systemic allergic response is also called [[anaphylaxis]]; multiple systems can be affected including the [[digestive system]], the [[respiratory system]], and the [[circulatory system]]. Depending of the rate of severity, it can cause [[cutaneous]] reactions, bronchoconstriction, [[edema]], [[hypotension]], [[coma]] and even [[death]]. This type of reaction can be triggered suddenly or the onset can be delayed. The severity of this type of allergic response often requires injections of [[epinephrine]], through a device known as the [[Epi-Pen]] auto-injector. The nature of [[anaphylaxis]] is such that the reaction can seemingly be subsiding, but may recur throughout a prolonged period of time.
[[Hay fever]] is one example of an exceedingly common{{Fact|date=March 2007}} minor allergy — large percentages of the population suffer from hayfever symptoms in response to [[wiktionary:airborne|airborne]] [[pollen]]. [[Asthma]]tics are often allergic to [[house dust mite|dust mites]]. Apart from ambient allergens, allergic reactions can be result from [[foods]], [[insect stings]] and reactions to [[medication]]s.
There are several methods for the diagnosis and assessment of allergies.
===Skin testing===
The typical and most simple{{Fact|date=March 2007}} method of diagnosis and monitoring of Type I Hypersensitivity is by skin testing, also known as "scratch testing" and "prick testing" due to the series of pricks and/or scratches made into the patient's skin. Small amounts of suspected allergens and/or their extracts (pollen, grass, mite proteins, peanut extract, etc.) are introduced to sites on the skin marked with pen or dye (the ink/dye should be carefully selected, lest it cause an allergic response itself). The allergens are either injected intradermally or into small scratchings made into the patient's skin, often with a [[lancet]]. Common areas for testing include the inside forearm and the back. If the patient is allergic to the substance, then a visible inflammatory reaction will usually occur within 30 minutes. This response will range from slight reddening of the skin to full-blown [[hives]] in extremely sensitive patients.
After performing the skin test and receiving results, the doctor may apply a [[steroid cream]], also known as [[corticosteriods]], to the test area to reduce discomfort (such as [[itch]]ing and [[inflammation]]){{Fact|date=March 2007}}.
====Problems with skin test====
The skin prick test is the most preferred means of testing because of its simplicity, economic implications and its accuracy relative to the other test available. Some people may display a delayed-type hypersensitivity (DTH) reaction which can occur up 6 hours after application of the allergen and last up to 72 hours. These reactions are from IgG antibodies and are called Arthus reactions. They usually are easily treated with anti-inflammatory creams and should not cause reason for alarm.
These types of reactions are fairly common and easily interpreted by a board certified allergist who is well versed in immunology.
Additionally, there are as many false negatives as there are false positives{{Fact|date=March 2007}} for the skin prick testing. The result is that people may be allergic to items that they did not show to be allergic to, and others may not be allergic to the item that had a positive reaction.
Testing for food allergies for peanuts, tree nuts and shellfish can be a delicate process. If a serious reaction has already been experienced most allergists will prefer a blood test to the skin prick to avoid the possibility of another allergic reaction.
One concern with the application of previously un-encountered insect venom allergen is the theoretical possibility that this exposure can actually sensitize one to these allergen, causing the inception of a new allergy in susceptible individuals{{Fact|date=March 2007}}.
Skins tests also are not always able to pinpoint a patient's specific allergies if the patient has significant skin conditions affecting the area of skin to be tested or a person has not avoided antihistamines of all types for the required period of time necessary for them to leave the body prior to testing.
Skin prick test for food allergens are very accurate if negative, if positive they must be correlated with elimination-challenge diets and clinical histories.
===Blood testing===
This kind of testing is also known as a "total IgE count". In order to qualify type I hypersensitivity, this method measures the amount of serum IgE contained within the patient's [[blood plasma|serum]]. This can be determined through the use of radiometric and colormetric [[immunoassay]]s. Even the levels the amount of IgE specific to certain allergens can be measured through use of the [[RAST test|radioallergosorbent test]] (RAST).
There are limited mainstream medical treatments for allergies{{Fact|date=March 2007}}. Probably the most important factor in rehabilitation is the removal of sources of allergens from the home environment, and avoiding environments in which contact with allergens is likely.
[[Hyposensitization]] is a form of [[immunotherapy]] where the patient is gradually [[vaccination|vaccinated]] against progressively larger doses of the allergen in question. This can either reduce the severity or eliminate hypersensitivity altogether. It relies on the progressive skewing of [[IgG]] ("the blocking antibody") production, as opposed to the excessive IgE production seen in hypersensitivity type I cases. Delivery can occur via allergy injection, or [[sublingual immunotherapy]] - allergy drops taken under the tongue. Though not commonly offered in the U.S., sublingual immunotherapy is gaining attention internationally and is very common in Europe{{Fact|date=March 2007}}.
A second form of immunotherapy involves the intravenous injection of [[monoclonal antibody|monoclonal]] anti-IgE antibodies. These bind to free and B-cell IgE, signalling such sources for destruction. They do not bind to IgE already bound to the Fc receptor on [[basophil]]s and mast cells as this would stimulate the allergic inflammatory response. The first agent in this class is [[omalizumab]].
An experimental treatment, [[enzyme potentiated desensitization]] (EPD), has been tried with some success but has yet to achieve widespread use{{Fact|date=March 2007}}. EPD uses dilutions of allergen and an enzyme, [[beta-glucuronidase]], to which T-regulatory lymphocytes respond by favouring desensitization, or down-regulation, rather than sensitization. EPD is also under development for the treatment of [[autoimmune diseases]].
Several [[antagonism|antagonistic]] drugs are used to block the action of allergic mediators, preventing activation of cells and degranulation processes. They include [[antihistamine]]s, [[cortisone]], [[epinephrine]] (adrenaline), [[theophylline]] and [[Cromolyn sodium]]. These drugs help alleviate the symptoms of allergy but play little role in chronic alleviation of the disorder. They can play an imperative role in the acute recovery of someone suffering from anaphylaxis, which is why those allergic to [[bee]] stings, [[peanut]]s, [[nut (fruit)|nuts]], and [[shellfish]] often carry an adrenaline needle with them at all times.
===Alternative therapies===
In [[alternative medicine]], a number of treatment modalities are considered effective by its practitioners in the treatment of allergies, particularly [[naturopathic]], [[herbal medicine]], [[homeopathy]], [[traditional Chinese medicine]] and [[kinesiology]]. These modalities are frequently offered as treatment for those seeking additional help when mainstream medicine has failed to provide adequate relief from allergy symptoms{{Fact|date=March 2007}}. However, mainstream physicians{{who}} maintain that these claims lack a scientific basis and warn that the efficacy of such treatments is only supported by anecdotal evidence.
The pathophysiology of allergic responses can be divided into two phases; firstly the acute response, which can then either subside or progress into a "late phase response" which can substantially prolong the symptoms of a response.
===Acute response===
The difference between a type I hypersensitivity reaction against an allergen to the normal [[humoral response]] against a foreign body is that [[plasma cell]]s secrete IgE as opposed to either IgM (against novel antigens) or IgG (against immunized antigens). IgE binds to Fc receptors on the surface of mast cells and basophils, both involved in the acute inflammatory response.
When IgE is first secreted it binds to the Fc receptors on a mast cell or basophil, and such an IgE-coated cell is said to be sensitized to the allergen in question. A later exposure by the same allergen causes reactivation of these IgE, which then signals for the [[degranulation]] of the sensitized mast cell or basophil. There is now strong evidence that mast cells and basophils require costimulatory signals for degranulation in vivo, derived from GPCRs such as chemokine receptors. These granules release [[histamine]] and other inflammatory chemical mediators ([[cytokine]]s, [[interleukin]]s, [[leukotriene]]s, and [[prostaglandin]]s) into the surrounding tissue causing several systemic effects, such as [[vasodilation]], [[mucous]] secretion, [[nerve]] stimulation and [[smooth muscle]] contraction. This results in the previously described symptoms of [[rhinorrhea]], itchiness, [[dyspnea]], and [[anaphylaxis]]. Depending on the individual, allergen, and mode of introduction, the symptoms can be system-wide ([[calliscal anaphylaxis]]), or localised to particular body systems (for example, [[asthma]] to the respiratory system; [[eczema]] to the [[dermis]]).
===Late-phase response===
After the chemical mediators of the acute response subside, late phase responses can often occur. This is due to the migration of other [[leukocyte]]s such as [[neutrophil]]s, [[lymphocyte]]s, [[eosinophil]]s and [[macrophage]]s to the initial site. The reaction is usually seen 4-6 hours after the original reaction and can last from 1-2 days. Cytokines from mast cells may also play a role in the persistence of long-term effects. Late phase responses seen in [[asthma]] are slightly different from those seen in other allergic responses.
==Basis and cause==
The exact cause of the IgE malfunctions that result in allergic reactions are not always apparent{{Fact|date=March 2007}}, however, and several arguments{{who}} from genetic-basis, environmental-basis and intermediate proponents exist with varying validity and acceptance.
===Genetic basis===
There is much evidence{{Fact|date=March 2007}} to support the genetic basis of allergy. Allergic parents are more likely to have allergic children, and their allergies are likely to be stronger than those from non-allergic parents. However some allergies are not consistent along genealogies with parents being allergic to peanuts, but having children allergic to ragweed, or siblings not sharing the same allergens.
Ethnicity has also been shown to play a role in some allergies. Interestingly, in regard to asthma, it has been suggested{{who}} that different genetic loci are responsible for asthma in people of Caucasian, Hispanic, Asian, and African origins. It has also been suggested{{who}} that there are both general atopy genes and tissue-specific allergy genes that target the allergic response to specific mucosal tissues. Potential disease associated alleles include both coding region variation and SNPs. Caucasian people seem to have the most asthma{{Fact|date=March 2007}}.
===Relationship with parasites===
Some recent research has also begun to show that some kinds of common parasites, such as intestinal worms (e.g. [[hookworm]]s), secrete [[immunosuppressant]] chemicals into the gut wall and hence the bloodstream which prevent the body from attacking the parasite. This gives rise to a new slant on the "[[hygiene hypothesis]]" — that co-evolution of man and parasites has in the past led to an immune system that only functions correctly in the presence of the parasites. Without them, the immune system becomes unbalanced and oversensitive. Gutworms and similar parasites are present in untreated drinking water in undeveloped countries, and in developed countries until the routine chlorination and purification of drinking water supplies. This also coincides with the time period in which a significant rise in allergies has been observed.{{Fact|date=February 2007}} So far, there is only sporadic evidence to support this hypothesis — one scientist who suffered from seasonal allergic rhinitis (hayfever) infected himself with gutworms and was immediately 'cured' of his allergy with no other ill effects.{{Fact|date=February 2007}} Full clinical trials have yet to be performed however. It may be that the term 'parasite' could turn out to be inappropriate, and in fact a hitherto unsuspected [[symbiosis]] is at work.{{Fact|date=February 2007}}
==Increasing prevalence==
<!--Is this section focused on IgE mediated immunity-->
There has been a notable increase in the commonness of allergies in the past decades{{Fact|date=March 2007}}, and there are multiple hypotheses explaining this phenomenon. This is in part because we know what they are, in contrast to earlier humans who would think that the symptoms pointed towards a non-important illness.
===Increasing use of chemicals===
One theory is the exponential use and abuse of chemicals in affluent nations since the second world war. Vast numbers of chemicals are introduced into our indoor and outdoor environments with little or no testing{{Fact|date=March 2007}} regarding their toxicity to living beings. Many believe{{who}} that air quality is getting worse rather than better, particularly if one considers indoor air quality as well as outdoor. (Indoor air quality has become significantly worse since building codes changed in the [[1970s]] to make buildings more air-tight to conserve energy. This affects buildings built since that time.) Adverse reactions to toxins vary considerably from one person to another, and can involve extremes in symptoms including the [[nervous system|neurological]] and [[endocrine system]]s as well as the more commonly recognized allergy symptoms listed above.
In 2004, a joint Swedish-Danish research team found a very strong link between allergies in children and the [[phthalates]] DEHP and BBzP, commonly used in [[polyvinyl chloride|PVC]].<!--
--><ref>{{cite journal | author = Bornehag C, Sundell J, Weschler C, Sigsgaard T, Lundgren B, Hasselgren M, Hägerhed-Engman L | title = The association between asthma and allergic symptoms in children and phthalates in house dust: a nested case-control study. | journal = Environ Health Perspect | volume = 112 | issue = 14 | pages = 1393-7 | year = 2004 | id = PMID 15471731 | url=}}</ref> Allergies are also viewed by some medical practitioners as a negative consequence of the use and abuse of [[antibiotics]] and [[vaccination]]s. This mainstream Western approach to treatment and prevention of infectious disease has been used in the more affluent world for a longer period of time than in the rest of the world, hence the much greater commonality of allergies there. It is hypothesized that use of antibiotics and vaccination affect the [[immune system]], and that allergies are a dysfunctional immune response.
===The "hygiene hypothesis"===
Another theory that has been gaining strength{{Fact|date=March 2007}} is the "[[hygiene hypothesis]]". This theory maintains{{Fact|date=March 2007}} that since children in more affluent countries are leading a cleaner and cleaner life (less exposure to dirt, extra use of disinfectants, etc), their immune systems have less exposure to [[parasite]]s and other pathogens than children in other countries or in decades past. Their immune systems may, therefore, have many "loaded guns", cells which might have targeted, say, the intestinal worms that no longer cause trouble in affluent neighbourhoods. Having no reasonable target, these cells inadvertently become activated by environmental [[antigen]]s that might only cause minor reactions in others. It is the symptoms of this exaggerated response that is seen as the allergic reaction.
Many common allergies such as asthma have seen huge increases{{Fact|date=March 2007}} in the years since the second world war, and many studies{{Fact|date=March 2007}} appear to show a correlation between this and the increasingly affluent and clean lifestyles in the [[Western world|West]]. This is supported by studies{{Fact|date=March 2007}} in less developed countries that do not enjoy western levels of cleanliness, and similarly do not show western levels of incidences of asthma and other allergies. During this same period, air quality, at one time considered the "obvious" cause of asthma, has shown a considerable improvement. This has led some researchers{{who}} to conclude that it is our "too clean" upbringing that is to blame for the lack of immune system stimulation in early childhood.
So far the evidence to support this theory is limited{{Fact|date=March 2007}}. One supporting fact{{Fact|date=March 2007}} is that many [[Han Chinese|Chinese]] will develop [[hay fever]] after moving into the [[USA]] for three or more years. However, contradictory examples also exist{{Fact|date=March 2007}}.
==Common allergens==
[[Image:Misc pollen.jpg|thumb|250px|right|[[Scanning electron microscope|SEM]] of miscellaneous plant pollens. Pollens are very common allergens.]]
[[Image:House Dust Mite.jpg|thumb|250px|right|The house [[dust mite]]; its feces and [[chitin]] are common allergens around the home]]
In addition to foreign proteins found in foreign serum (from [[blood transfusion]]s) and [[vaccine]]s, common allergens include:
* [[Plant]] [[pollen]]s ([[Hay fever]]):
** [[Ryegrass|Rye grass]]
** [[Ragweed]]
** [[Timothy-grass|Timothy grass]]
** [[Birch]] trees
* [[Mold]] [[spore]]s
* [[Medication|Drugs]]:
** [[Penicillin]]s
** [[Sulfonamides]]
** [[Salicylate]]s (also found naturally in numerous [[fruit]]s)
** [[Local anaesthetic]]s
* Foods ([[food allergy]]):
** [[Nut (fruit)|Nut]]s (2 separate cases)
*** [[Peanut]]s
*** [[Nut (fruit)|Nut]]s
** [[Sesame]]
** [[Seafood]]
** [[Egg (food)|Egg]] (typically [[Egg white|albumen]], the white)
** [[Pea]]s, [[bean]]s, [[peanut]]s, [[soybean]]s and other [[legume]]s
** [[Soy allergy|Soy]]
** [[Milk allergy|Milk]]
** [[Wheat]]
** [[Maize]] (corn)
* [[Insect]] stings:
** [[Bee]] sting [[venom (poison)|venom]]
** [[Wasp]] sting venom
* Animal products (animal allergy):
** [[Fur|Animal hair]] and [[dander]]
** [[Cockroach]] calyx
** [[Dust mite]] excretion
* Other
** [[Latex]]
== See also ==
* [[Allergic Inflammation|Allergic inflammation]]
* [[Anaphylaxis]]
* [[Atopy]]
* [[Food allergy]]
* [[Hay fever]]
* [[Hypoallergenic]]
* [[Multiple chemical sensitivity]] (MCS).
* [[Soy allergy]]
===Other sources===
* {{cite book | author=Goldsby RA, Kindt TK, Osborne BA and Kuby J | title=Immunology | year=2003 | edition=5th Edition | publisher= W.H. Freeman and Company | location=New York | id=ISBN 0-7167-4947-5}}
* {{cite journal | author = Ono S | title = Molecular genetics of allergic diseases. | journal = Annu Rev Immunol | volume = 18 | issue = | pages = 347-66 | year =2000 | id = PMID 10837062}}
==External links==
* [ American Academy of Allergy, Asthma & Immunology]
* [ Asthma and Allergy Foundation of America] – patient advocacy organization
* [ Case Studies in Environmental Medicine (CSEM): Environmental Triggers of Asthma] – Agency for Toxic Substances and Disease Registry, U.S. Department of Health and Human Services.
[[Category:Immune system]]

Latest revision as of 19:59, December 12, 2012

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