Allergy

Allergy is a disorder of the immune system which is a form of hypersensitivity. Allergic reactions occur to normally harmless 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 attacks, food allergies, and reactions to the venom of stinging insects such as wasps and bees¹.
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.

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Signs and symptoms

Common Symptoms of Allergy

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 eyes². 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 wheezing³.

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, 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 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 eczema⁴.

Cause

Risk factors for allergy can be placed in two general categories, namely host and environmental factors⁵. 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 changes⁶.

• Food allergens
  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 about one in fifty children but are frequently outgrown by children when they reach age five. Typically the sensitivity is to proteins in the yolk, rather than the white⁷.
  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. 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, and chemical additives.
  
  
• Non-food protein allergens
  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 health care workers is higher, between seven and ten percent. Researchers attribute this higher level to the exposure of health care workers to areas with significant airborne latex allergens, such as operating rooms, intensive care units, and dental suites. These latex-rich environments may sensitize health care workers who regularly inhale allergenic proteins⁸.
  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 reactions⁸.
  Latex and banana sensitivity may cross-react; furthermore, patients with latex allergy may also have sensitivities to avocado, kiwi, and chestnut. Researchers suspect that the cross-reactivity of latex with banana, avocado, kiwi, and chestnut probably occurs because latex proteins are structurally homologous with some plant proteins⁸.
  
  Toxins interacting with 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 streaking⁹.
  
  
• 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 children¹⁰, 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 not¹⁰.
  The risk of allergic sensitization and the development of allergies varies with age, with young children most at risk¹¹.
  
  
• Hygiene hypothesis
  According to the hygiene hypothesis, proposed by David P. Strachan, allergic diseases are caused by individuals living in too sterile an environment not being 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 response¹².
  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 world¹³. Longitudinal studies in the third world demonstrate an increase in immunological disorders as a country grows more affluent and, presumably, cleaner¹⁴. The use of antibiotics in the first year of life has been linked to asthma and other allergic diseases¹⁵.

Diagnosis

Many allergies share common symptoms with other diseases. Before a diagnosis of allergic disease can be confirmed, other possible causes of the presenting symptoms have to be eliminated. The following methods are used for diagnosing allegies.

1. Skin testing
   
   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. 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. Interpretation of the results of the skin prick test is normally done by allergists on a scale of severity.
2. Blood testing
   Various blood allergy testing methods are also available for detecting allergy to specific substances.

Treatment

Traditional treatment and management of allergies consisted simply of avoiding the allergen in question or otherwise reducing exposure. For instance, people with cat allergies were encouraged to avoid them. However, while avoidance of allergens may reduce symptoms and avoid life-threatening anaphylaxis, it is difficult to achieve for those with pollen or similar air-borne allergies. Nonetheless, strict avoidance of allergens is still considered a useful treatment method, and is often used in managing food allergies.

Several antagonistic drugs are used to block the action of allergic mediators, or to prevent activation of cells and degranulation processes. These include antihistamines, glucocorticoids, epinephrine (adrenaline), theophylline and cromolyn sodium.

Desensitization is a treatment in which the patient is gradually vaccinated with progressively larger doses of the allergen in question. This can either reduce the severity or eliminate hypersensitivity altogether.

Allergy shot treatment is the closest thing to a ‘cure’ for allergic symptoms. This therapy requires a long-term commitment.

In alternative medicine, a number of allergy treatments are in use. However, there no convincing evidence that supports the use of homeopathic treatments¹⁶.

References

1. Kay AB (2000), "Overview of 'allergy and allergic diseases: with a view to the future'", Br. Med. Bull., 56, 4, 843–64.
2. Bope, Edward T.; Rakel, Robert E. (2005), Conn's Current Therapy, W.B. Saunders Company,Philadelphia, PA, 880.
3. Holgate ST (1998), "Asthma and allergy--disorders of civilization?". QJM, 91, 3, 171–84.
4. Brehler R, Kütting B (2001), "Natural rubber latex allergy: a problem of interdisciplinary concern in medicine", Arch. Intern. Med., 161, 8, 1057–64.
5. Grammatikos AP (2008), "The genetic and environmental basis of atopic diseases", Ann. Med., 40, 7, 482–95.
6. Janeway, Charles; Paul Travers, Mark Walport, and Mark Shlomchik (2001),"Immunobiology, Fifth Edition", Garland Science, New York and London.
7. Sicherer SH, Leung DY (2007), "Advances in allergic skin disease, anaphylaxis, and hypersensitivity reactions to foods, drugs, and insects", J. Allergy Clin. Immunol., 119, 6, 1462–9.
8. Gordon L. Sussman, MD, FRCP; Donald H. Beezhold. "Allergy to Latex Rubber", Annals of Internal Medicine., 22, 1, 43–46.
9. DermAtlas, Online Dermatology Image Library, Accessed May 20, 2009.
10. De Swert LF (1999), "Risk factors for allergy", Eur. J. Pediatr., 158, 2, 89–94.
11. Croner S (1992), "Prediction and detection of allergy development: influence of genetic and environmental factors", J. Pediatr., 121, 5 Pt 2, S58–63.
12. Edward Willett,"The Hygiene Hypothesis", Accessed May 15,2009.
13. Gibson PG, Henry RL, Shah S, Powell H, Wang H (September 2003), "Migration to a western country increases asthma symptoms but not eosinophilic airway inflammation", Pediatr. Pulmonol., 36, 3, 209–15.
14. Addo-Yobo EO, Woodcock A, Allotey A, Baffoe-Bonnie B, Strachan D, Custovic A (February 2007), "Exercise-induced bronchospasm and atopy in Ghana: two surveys ten years apart", PLoS Med., 4, 2, e70. Accessed May 19, 2009.
15. Marra F, Lynd L, Coombes M "et al." (2006), "Does antibiotic exposure during infancy lead to development of asthma?: a systematic review and metaanalysis", Chest, 129, 3, 610–8.
16. Altunç U, Pittler MH, Ernst E (2007), "Homeopathy for childhood and adolescence ailments: systematic review of randomized clinical trials", Mayo Clin. Proc., 82, 1, 69–75.

External links

• American Academy of Allergy, Asthma & Immunology
• Allergy & Asthma Network Mothers of Asthmatics