AskDefine | Define allergic

Dictionary Definition

allergic adj
1 characterized by or caused by allergy; "an allergic reaction"
2 having an allergy or peculiar or excessive susceptibility (especially to a specific factor); "allergic children"; "hypersensitive to pollen" [syn: hypersensitive, hypersensitized, hypersensitised, sensitized, sensitised, supersensitive, supersensitized, supersensitised]

User Contributed Dictionary

English

Pronunciation

Etymology

Adjective

  1. Of or pertaining to allergy.
    I am allergic to cats.
  2. Having an allergy.
  3. Excessively sensitive; susceptible.

Related terms

Translations

pertaining to allergy
having an allergy
excessively sensitive

Extensive Definition

Allergy is a disorder of the immune system often also referred to as atopy. Allergic reactions occur to environmental substances known as allergens; these reactions are acquired, predictable and rapid. Strictly, allergy is one of four forms of hypersensitivity and is called type I (or immediate) hypersensitivity. It is characterized by excessive activation of certain white blood cells called mast cells and basophils by a type of antibody known as IgE, resulting in an extreme inflammatory response. Common allergic reactions include eczema, hives, hay fever, asthma, food allergies, and reactions to the venom of stinging insects such as wasps and bees.
Mild allergies like hay fever are highly prevalent in the human population and cause symptoms such as allergic conjunctivitis, itchiness, and runny nose. Allergies can play a major role in conditions such as asthma. In some people, severe allergies to environmental or dietary allergens or to medication may result in life-threatening anaphylactic reactions and potentially death.
A variety of tests now exist to diagnose allergic conditions; these include testing the skin for responses to known allergens or analyzing the blood for the presence and levels of allergen-specific IgE. Treatments for allergies include allergen avoidance, use of anti-histamines, steroids or other oral medications, immunotherapy to desensitize the response to allergen, and targeted therapy.

Classification and history

The concept "allergy" was originally introduced in 1906 by the Viennese pediatrician Clemens von Pirquet, after he noted that some of his patients were hypersensitive to normally innocuous entities such as dust, pollen, or certain foods. Pirquet called this phenomenon "allergy" from the Greek words allos meaning "other" and ergon meaning "work". Historically, all forms of hypersensitivity were classified as allergies, and all were thought to be caused by an improper activation of the immune system. Later, it became clear that several different disease mechanisms were implicated, with the common link to a disordered activation of the immune system. In 1963, a new classification scheme was designed by Philip Gell and Robin Coombs that described four types of hypersensitivity reactions, known as Type I to Type IV hypersensitivity. With this new classification, the word "allergy" was restricted to only type I hypersensitivities (also called immediate hypersensitivity), which are characterized as rapidly developing reactions.
A major breakthrough in understanding the mechanisms of allergy was the discovery of the antibody class labeled immunoglobulin E (IgE) - Kimishige Ishizaka and co-workers were the first to isolate and describe IgE in the 1960s.

Signs and symptoms

Many allergens, such as dust or pollen, are airborne particles. In these cases, symptoms arise in areas in contact with air, such as eyes, nose and lungs. For instance, allergic rhinitis, also known as hay fever, causes irritation of the nose, sneezing, and itching and redness of the eyes. Inhaled allergens can also lead to asthmatic symptoms, caused by narrowing of the airways (bronchoconstriction) and increased production of mucus in the lungs, shortness of breath (dyspnea), coughing and wheezing.
Aside from these ambient allergens, allergic reactions can result from foods, insect stings, and reactions to medications like aspirin and antibiotics such as penicillin. Symptoms of food allergy include abdominal pain, bloating, vomiting, diarrhoea, itchy skin, and swelling of the skin during hives. Food allergies rarely cause respiratory (asthmatic) reactions, or rhinitis. Insect stings, antibiotics, and certain medicines produce a systemic allergic response that is also called anaphylaxis; multiple organ 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, sometimes through a device known as the Epi-Pen auto-injector. The nature of anaphylaxis is such that the reaction can seem to be subsiding, but may recur throughout a prolonged period of time. Skin allergies frequently cause rashes, or swelling and inflammation within the skin, in what is known as a "wheal and flare" reaction characteristic of hives and angioedema.

Cause

Risk factors for allergy can be placed in two general categories, namely host and environmental factors. Host factors include heredity, sex, race, and age, with heredity being by far the most significant. There have been recent increases in the incidence of allergic disorders, however, that cannot be explained by genetic factors alone. The four main environmental candidates are alterations in exposure to infectious diseases during early childhood, environmental pollution, allergen levels, and dietary changes.

Genetic basis

Allergic diseases are strongly familial: identical twins are likely to have the same allergic diseases about 70% of the time; the same allergy occurs about 40% of the time in non-identical twins. Allergic parents are more likely to have allergic children, and their allergies are likely to be more severe than those from non-allergic parents. Some allergies, however, are not consistent along genealogies; parents who are allergic to peanuts may have children who are allergic to ragweed. It seems that the likelihood of developing allergies is inherited and related to an irregularity in the immune system, but the specific allergen is not. Several studies have shown that IgE levels are highest in childhood and fall rapidly between the ages of 10 and 30 years. Overall, boys have a higher risk of developing allergy than girls, Sex differences tend to decrease in adulthood.

Environmental factors

International differences have been associated with the number of individuals within a population that suffer from allergy. Allergic diseases are more common in industrialized countries than in countries that are more traditional or agricultural, and there is a higher rate of allergic disease in urban populations versus rural populations, although these differences are becoming less defined.
Exposure to allergens, especially in early life, is an important risk factor for allergy. Alterations in exposure to microorganisms is the most plausible explanation, at present, for the increase in atopic allergy. Endotoxin exposure reduces release of inflammatory cytokines such as TNF-α, IFNγ, interleukin-10, and interleukin-12 from white blood cells (leukocytes) that circulate in the blood. Certain microbe-sensing proteins, known as Toll-like receptors, found on the surface of cells in the body are also thought to be involved in these processes.
Gutworms and similar parasites are present in untreated drinking water in developing countries, and were present in the water of developed countries until the routine chlorination and purification of drinking water supplies. Recent research has shown that some common parasites, such as intestinal worms (e.g. hookworms), secrete chemicals into the gut wall (and hence the bloodstream) that suppress the immune system and prevent the body from attacking the parasite. This gives rise to a new slant on the hygiene hypothesis theory — that co-evolution of man and parasites has led to an immune system that only functions correctly in the presence of the parasites. Without them, the immune system becomes unbalanced and oversensitive. In particular, research suggests that allergies may coincide with the delayed establishment of gut flora in infants. However, the research to support this theory is conflicting, with some studies performed in China and Ethiopia showing an increase in allergy in people infected with intestinal worms. It may be that the term 'parasite' could turn out to be inappropriate, and in fact a hitherto unsuspected symbiosis is at work. 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, although they are still caused by release of mediators from eosinophils, and are still dependent on activity of TH2 cells.

Diagnosis

Before a diagnosis of allergic disease can be confirmed, the other possible causes of the presenting symptoms should be carefully considered. Vasomotor rhinitis, for example, is one of many maladies that shares symptoms with allergic rhinitis, underscoring the need for professional differential diagnosis. Once a diagnosis of asthma, rhinitis, anaphylaxis, or other allergic disease has been made, there are several methods for discovering the causative agent of that allergy.

References

allergic in Arabic: حساسية
allergic in Asturian: Alerxa
allergic in Bulgarian: Алергия
allergic in Catalan: Al·lèrgia
allergic in Czech: Alergie
allergic in Danish: Allergi
allergic in German: Allergie
allergic in Estonian: Allergia
allergic in Spanish: Alergia
allergic in Esperanto: Alergio
allergic in Basque: Alergologia
allergic in Persian: آلرژی
allergic in French: Allergie
allergic in Korean: 알레르기
allergic in Croatian: Alergija
allergic in Ido: Alergio
allergic in Indonesian: Alergi
allergic in Interlingua (International Auxiliary Language Association): Allergia
allergic in Italian: Allergia
allergic in Hebrew: אלרגיה
allergic in Lithuanian: Alergija
allergic in Hungarian: Allergia
allergic in Macedonian: Алергија
allergic in Malay (macrolanguage): Alergi
allergic in Mongolian: Харшил
allergic in Dutch: Allergie
allergic in Japanese: アレルギー
allergic in Norwegian: Allergi
allergic in Uzbek: Allergiya
allergic in Pushto: الرژي
allergic in Polish: Alergia
allergic in Portuguese: Alergia
allergic in Romanian: Alergie
allergic in Russian: Аллергия
allergic in Simple English: Allergy
allergic in Slovak: Alergia
allergic in Serbian: Алергија
allergic in Finnish: Allergia
allergic in Swedish: Allergi
allergic in Tamil: ஒவ்வாமை
allergic in Thai: โรคภูมิแพ้
allergic in Turkish: Alerji
allergic in Ukrainian: Алергія
allergic in Venetian: Ałergia
allergic in Chinese: 过敏

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