Online Encyclopedia
Allergy
See Allergy/proposed change for a proposed rewrite of this article
An allergy is an immune system response to something which is often not directly dangerous to the body. The word derives from the Greek words allos meaning "other" and ergon meaning "reaction" or "reactivity".
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History
The term and concept of "allergy" were coined by Viennese pediatrician Baron Clemens von Pirquet in 1906. He observed that the symptoms of some of his patients might have been a response to outside allergens such as dust, pollen, or certain foods.
Signs and symptoms
Allergy is characterised by a local or systemic inflammatory response to allergens. Local symptoms are:
- Nose: swelling of the nasal mucosa (allergic rhinitis)
- Eyes: redness and itching of the conjunctiva (allergic conjunctivitis )
- Airways: bronchoconstriction , wheezing and dyspnoea, sometimes outright attacks of asthma
- Skin: various rashes, such as eczema, hives (urticaria) and contact dermatitis.
Systemic allergic response is also called anaphylaxis. Depending of the rate of severity, it can cause cutaneous reactions, bronchoconstriction , oedema, hypotension, coma and death.
Hay fever is one example of an exceedingly common minor allergy - large percentages of the population suffer from hayfever symptoms in response to airborne pollen. Asthmatics are often allergic to dust mites. Apart from ambient allergens, allergic reactions can be due to medications.
Diagnosis
Generally, diagnosis of allergy is based on the medical history and a brief physical examination. When there is diagnostic uncertainty, or when hyposensibilization is considered, testing for specific antigens can be useful.
Laboratory tests: full blood count (occasionally there is an increased eosinophil count), erythrocyte sedimentation rate or C-reactive protein, renal function and electrolytes. Total IgE (an immunoglobulin important in allergy) and a RAST (radioallergosorbent test) can identify allergies.
Specific tests:
- Type 1 allergies (the commonest, see below) are often diagnosed with skin-prick testing. This involves making a small scratch on the skin and applying a number of suspected allergens. A true-positive (histamine) and a true-negative (saline) are usually applied as controls.
- Type 4 allergies (contact eczema ) need patch testing, which involves applying the allergen on a patch of skin under a large plastic plaster.
Treatment
The only known mainstream medicine treatment for allergy is hyposensibilization. Other treatments are symptomatic: these include antihistamines and cortisone, can have the effect of reducing the symptoms.
In the 1960s, Dr. Len McEwen in the United Kingdom developed a treatment for allergies known as enzyme potentiated desensitization , or EPD. EPD uses much lower doses of antigens than conventional treatment, with the addition of an enzyme. EPD is available in the United Kingdom and Canada, and was available in the United States until 2001, when the Food and Drug Administration revoked its approval for an investigative study being performed. Since that time an American counterpart to EPD, known as Low Dose Antigens , or LDA, has been formulated from components approved by the FDA, and is available for treatment from a small number of doctors in the United States. EPD (and LDA) is still considered experimental by many mainstream doctors and medical insurance companies, and many doubt that it is more effective than a placebo.
In alternative medicine, a number of treatment modalities are considered effective by its practitioners in the treatment of allergies, particularly traditional Chinese medicine. However, none of these have been backed up by good quality evidence. On the contrary, they are generally criticised by mainstream medical researchers to be mainly supported by anecdotes, which make them suspectible to be placebos.
Pathophysiology
Immunopathology
Recent research has shed some light on the mechanism by which many allergies operate, but this is by no means the whole story; some causative factors and allergenic pathways have yet to be explained.
There are four (some argue five) types of immune hypersensitivity reactions(Gell & Coombs-classification):
- Type 1: IgE-mediated (anaphylactoid)
- Type 2: IgG-mediated
- Type 3: Arthus reaction (serum sickness)
- Type 4: Cellular immunity (T cell-mediated)
Certain individuals appear predisposed to the development of allergies. The combination of allergic rhinitis and conjunctivitis, atopic eczema and asthma is termed the "atopic syndrome".
It is presumed that predisposed individuals develop Type 1 allergy against a number of very abundant allergens, such as the house dust mite excrement and grass pollen . All allergens are protein-based.
Allergy is immunologically a very complicated mechanism, but the main mechanism is the production of IgE antibodies against specific allergens. The antibodies are coated to mast cells, a type of cell present in epithelial tissues. When the allergen comes into contact with IgE, the mast cell degranulates and releases histamine and a number of other compounds that stimulate a local inflammatory reaction. As the skin and the nasal and conjunctival epithelia are most prone to come into contact with allergens, this is also the place where allergic symptoms develop most commonly. Food allergy is a similar process directed against food-borne allergens, leading to diarrhea and abdominal pain. The anaphylactoid reaction is very quick (seconds to minutes); any set of complaints lasting longer is probably due to another cause.
The hygiene hypothesis
One theory that has been gaining strength is the "hygiene hypothesis". This theory maintains 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 parasites and other pathogens than children in other countries or in decades past. Their immune systems may have therefore 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 antigens 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 in the years since the second world war, and many studies appear to show a correlation between this increasingly affluent and clean lifestyles in the west. This is supported by studies 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 lead some researchers to conclude that it is our "too clean" upbringing that is to blame for the lack of immune system stimulation in early childhood.
Alternative views
Another 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 regarding their toxicity to living beings. Many believe 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 and therefore to conserve energy. This affects buildings built since this time.) Adverse reactions to toxins vary considerably from one person to another, and can involve extremes in symptoms including the neurological and endocrine systems as well as the more commonly recognized allergy symptoms listed above.
Allergies are also viewed by some medical practitioners as a negative consequence of the use and abuse of antibiotics and vaccinations. This mainstream Western approach to treatment and prevention of infectious disease has been used in the more affluent "First world" for a longer period of time than in the rest of the world, hence the much greater commonality of allergies in the First world. It is hypothesized that use of antibiotics and vaccination affect the immune system, and that allergies are a dysfunctional immune response. There is however very little evidence to support this view.