Аllergy

An allergy is a hypersensitivity disorder of the immune system.  Allergic reactions occur when a person's immune system reacts to normally harmless substances in the environment. A substance that causes a reaction is called an allergen. These reactions are acquired, predictable, and rapid. Allergy is one of four forms of hypersensitivity and is formally called type I (or immediate) hypersensitivity. Allergic reactions are distinctive because of excessive activation of certain white blood cells called mast cells and basophils by a type of antibody called Immunoglobulin E (IgE). This reaction results in aninflammatory response which can range from uncomfortable to dangerous.

Mild allergies like hay fever are very common in the human population and cause symptoms such as red eyes, itchiness, and runny nose, eczema, hives,hay fever, or an asthma attack. 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 reactions called anaphylaxis. Food allergies, and reactions to the venom of stinging insects such aswasps and bees are often associated with these severe reactions. 

A variety of tests exist to diagnose allergic conditions. These include placing possible allergens on the skin and looking for a reaction such as swelling. Blood tests can also be done to look for an allergen-specific IgE.

Treatments for allergies include avoiding known allergens, use of medications such as anti-histamines that specifically prevent allergic reactions, steroidsthat modify the immune system in general, and medications such as decongestants that reduce the symptoms. Many of these medications are taken by mouth, though epinephrine, which is used to treat anaphylactic reactions, is injected. Immunotherapy uses injected allergens to desensitize the body's response.

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, 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, diarrhea, 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, therespiratory system, and the circulatory system.  Depending on 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 EpiPen or Twinject 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. 

Substances that come into contact with the skin, such as latex, are also common causes of allergic reactions, known as contact dermatitis or eczema.  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. 

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

One of the most common food allergies is a sensitivity to peanuts. Peanut allergies may be extremely severe, but can sometimes be outgrown by children school-age.  Tree nuts, including pecans, pistachios, pine nuts, and walnuts, are another common allergen. Sufferers may be sensitive to one, or many, tree nuts.  Also seeds, including sesame seeds and poppy seeds, contain oils where protein is present, which may elicit an allergic reaction. 

Egg allergies affect one to two percent of children but are outgrown by about two-thirds of children by the age of 5. The sensitivity is usually to proteins in the white rather than the yolk. 

Milk, from cows, goats, or sheep, is another common allergy-causing food, and many sufferers are also unable to tolerate dairy products such as cheese. Lactose intolerance, a common reaction to milk, is not in fact a form of allergy. A small portion of children with a milk allergy, roughly ten percent, will have a reaction to beef. Beef contains a small amount of protein that is present in cow's milk. 

Other foods containing allergenic proteins include soy, wheat, fish, shellfish, fruits, vegetables, spices, synthetic and natural colors, chicken, and chemical additives. 

Latex can trigger an IgE-mediated cutaneous, respiratory, and systemic reaction. The prevalence of latex allergy in the general population is believed to be less than one percent. In a hospital study, one in 800 surgical patients (0.125 percent) report latex sensitivity, although the sensitivity among healthcare workers is higher, between seven and ten percent. Researchers attribute this higher level to the exposure of healthcare workers to areas with significant airborne latex allergens, such as operating rooms, intensive-care units, and dental suites. These latex-rich environments may sensitize healthcare workers who regularly inhale allergenic proteins. 

The most prevalent response to latex is an allergic contact dermatitis, a delayed hypersensitive reaction appearing as dry, crusted lesions. This reaction usually lasts 48 to 96 hours. Sweating or rubbing the area under the glove aggravates the lesions, possibly leading to ulcerations.  Anaphylactic reactions occur most often in sensitive patients, who have been exposed to the surgeon's latex gloves during abdominal surgery, but other mucosal exposures, such as dental procedures, can also produce systemic reactions. 

Latex and banana sensitivity may cross-react; furthermore, patients with latex allergy may also have sensitivities to avocado, kiwifruit, and chestnut. These patients often have perioral itching and local urticaria. Only occasionally have these food-induced allergies induced systemic responses. Researchers suspect that the cross-reactivity of latex with banana, avocado, kiwifruit, and chestnut occurs because latex proteins are structurally homologous with some plant proteins.  

Another non-food protein reaction, urushiol-induced contact dermatitis, originates after contact with poison ivy, eastern poison oak, western poison oak, or poison sumac. Urushiol, which is not itself a protein, acts as a hapten and chemically reacts with, binds to, and changes the shape of integral membrane proteins on exposed skin cells. The immune system does not recognize the affected cells as normal parts of the body, causing a T-cell-mediated immune response.  Of these poisonous plants, sumac is the most virulent.  The resulting dermatological response to the reaction between urushiol and membrane proteins includes redness, swelling, papules, vesicles, blisters, and streaking. 

Estimates vary on the percentage of the population that will have an immune system response. Approximately 25 percent of the population will have a strong allergic response to urushiol. In general, approximately 80 percent to 90 percent of adults will develop a rash if they are exposed to .0050 milligrams (7.7×10−5 gr) of purified urushiol, but some people are so sensitive that it takes only a molecular trace on the skin to initiate an allergic reaction.  

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 alonggenealogies; 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. 

The risk of allergic sensitization and the development of allergies varies with age, with young children most at risk.  Several studies have shown that IgE levels are highest in childhood and fall rapidly between the ages of 10 and 30 years. The peak prevalence of hay fever is highest in children and young adults and the incidence of asthma is highest in children under 10.  Overall, boys have a higher risk of developing allergy than girls,  although for some diseases, namely asthma in young adults, females are more likely to be affected. Sex differences tend to decrease in adulthood. Ethnicity may play a role in some allergies; however, racial factors have been difficult to separate from environmental influences and changes due to migration.  It has been suggested that differentgenetic loci are responsible for asthma, to be specific, in people of European, Hispanic, Asian, and African origins.  

Allergic diseases are caused by inappropriate immunological responses to harmless antigens driven by a TH2-mediated immune response. Many bacteria and viruses elicit a TH1-mediated immune response, which down-regulates TH2 responses. The first proposed mechanism of action of the hygiene hypothesis stated that insufficient stimulation of the TH1 arm of the immune system lead to an overactive TH2 arm, which in turn led to allergic disease.  In other words, individuals living in too sterile an environment are not exposed to enough pathogens to keep the immune system busy. Since our bodies evolved to deal with a certain level of such pathogens, when it is not exposed to this level, the immune system will attack harmless antigens and thus normally benign microbial objects — like pollen — will trigger an immune response. 

The hygiene hypothesis was developed to explain the observation that hay fever and eczema, both allergic diseases, were less common in children from larger families, which were, it is presumed, exposed to more infectious agents through their siblings, than in children from families with only one child. The hygiene hypothesis has been extensively investigated by immunologists andepidemiologists and has become an important theoretical framework for the study of allergic disorders. It is used to explain the increase in allergic diseases that have been seen since industrialization, and the higher incidence of allergic diseases in more developed countries. The hygiene hypothesis has now expanded to include exposure to symbiotic bacteria and parasites as important modulators of immune system development, along with infectious agents.

Epidemiological data support the hygiene hypothesis. Studies have shown that various immunological and autoimmune diseases are much less common in the developing world than the industrialized world and that immigrants to the industrialized world from the developing world increasingly develop immunological disorders in relation to the length of time since arrival in the industrialized world. Longitudinal studies in the third world demonstrate an increase in immunological disorders as a country grows more affluent and, it is presumed, cleaner. The use of antibiotics in the first year of life has been linked to asthma and other allergic diseases. The use of antibacterial cleaning products has also been associated with higher incidence of asthma, as has birth by Caesarean section rather than vaginal birth.  

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 another 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 functions correctly only 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.  Clinical trials have been initiated to test the effectiveness of certain worms in treating some allergies.  It may be that the term 'parasite' could turn out to be inappropriate, and in fact a hitherto unsuspected symbiosis is at work. For more information on this topic, see Helminthic therapy.

In the early stages of allergy, a type I hypersensitivity reaction against an allergen encountered for the first time and presented by a professional Antigen-Presenting Cell causes a response in a type of immune cell called a TH2 lymphocyte, which belongs to a subset of T cells that produce a cytokine calledinterleukin-4 (IL-4). These TH2 cells interact with other lymphocytes called B cells, whose role is production of antibodies. Coupled with signals provided by IL-4, this interaction stimulates the B cell to begin production of a large amount of a particular type of antibody known as IgE. Secreted IgE circulates in the blood and binds to an IgE-specific receptor (a kind of Fc receptor called FcεRI) on the surface of other kinds of immune cells called mast cells and basophils, which are both involved in the acute inflammatory response. The IgE-coated cells, at this stage are sensitized to the allergen. 

If later exposure to the same allergen occurs, the allergen can bind to the IgE molecules held on the surface of the mast cells or basophils. Cross-linking of the IgE and Fc receptors occurs when more than one IgE-receptor complex interacts with the same allergenic molecule, and activates the sensitized cell. Activated mast cells and basophils undergo a process called degranulation, during which they release histamine and other inflammatory chemical mediators (cytokines, interleukins, leukotrienes, and prostaglandins) from their granules into the surrounding tissue causing several systemic effects, such asvasodilation, mucous secretion, nerve stimulation, and smooth muscle contraction. This results in rhinorrhea, itchiness, dyspnea, and anaphylaxis. Depending on the individual, allergen, and mode of introduction, the symptoms can be system-wide (classical anaphylaxis), or localized to particular body systems; asthma is localized to the respiratory system and eczema is localized to the dermis.  

After the chemical mediators of the acute response subside, late phase responses can often occur. This is due to the migration of other leukocytes such asneutrophils, lymphocytes, eosinophils and macrophages to the initial site. The reaction is usually seen 2–24 hours after the original reaction. 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.

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.

Effective management of allergic diseases relies on the ability to make an accurate diagnosis. Allergy testing can help confirm/rule out allergies and consequently reduce adverse reactions and limit unnecessary avoidance and medications.  Correct diagnosis, counseling and avoidance advice based on valid allergy test results will help reduce the incidence of symptoms, medications and improve quality of life.  For assessing the presence of allergen-specific IgE antibodies, you can use two different methods—a skin prick test or an allergy blood test. Both methods are recommended by the NIH guidelines and have similar diagnostic value in terms of sensitivity and specificity.

A healthcare provider can use the test results to identify the specific allergic triggers that may be contributing to the symptoms. Using this information, along with a physical examination and case history, the doctor can diagnose the cause of the symptoms and tailor treatments that will help the patient feel better. A negative result can help the doctor rule out allergies in order to consider other possible.

NIH Guidelines state that: "sIgE tests are useful for identifying foods potentially provoking IgE-mediated food-induced allergic reactions, and specified ‘‘cutoff’’ levels, defined as 95% predictive values, may be more predictive than skin prick tests of clinical reactivity in certain populations.” It further states, "sIgE tests are very useful for detecting the presence of sIgE antibodies, which indicates the presence of allergic sensitization. Fluorescence-labeled antibody assays have comparable sensitivity to that of skin prick tests, and the absolute levels of sIgE antibodies may directly correlate with the likelihood of clinical reactivity when compared with oral food challenges for the identification of foods provoking IgE mediated FA.” 

According to NICE Guidelines, skin prick tests and blood tests are equally cost-effective and health economic evidence show that both the IgE antibody test and the skin prick test were cost effective compared with no test  Also, earlier and more accurate diagnoses save cost due to reduced GP consultations, referrals to secondary care, misdiagnosis and emergency admissions. 

Allergy undergoes dynamic changes over time. Regular allergy testing of relevant allergens provides information on if and how patient management can be changed, in order to improve health and quality of life. Annual testing is often the practice for determining whether allergy to milk, egg, soy, and wheat have been outgrown and the testing interval is extended to 2 to 3 years for allergy to peanut, tree nuts, fish, and crustacean shellfish.  Results of follow-up testing can guide decision-making regarding whether and when it is safe to introduce or re-introduce allergenic food into the diet. 

Skin testing is also known as "puncture testing" and "prick testing" due to the series of tiny puncture or pricks 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). A small plastic or metal device is used to puncture or prick the skin. Sometimes, the allergens are injected "intradermally" into the patient's skin, with a needle and syringe. 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 a full-blown hive (called "wheal and flare") in more sensitive patients similar to a mosquito bite. Interpretation of the results of the skin prick test is normally done by allergists on a scale of severity, with +/- meaning borderline reactivity, and 4+ being a large reaction. Increasingly, allergists are measuring and recording the diameter of the wheal and flare reaction. Interpretation by well-trained allergists is often guided by relevant literature.  Some patients may believe they have determined their own allergic sensitivity from observation, but a skin test has been shown to be much better than patient observation to detect allergy. 

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