Allergy - Types, Signs & Symptoms, Test & Diagnosis, Prevention & Treatment

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What are allergies? Allergies, also known as allergic diseases, are a number of conditions caused by hypersensitivity of the immune system to typically harmless substances in the environment. Allergens may cause an allergic reaction when they come in contact with skin or the eye, when they are inhaled, eaten, or are injected. An allergic reaction can occur as part of a seasonal allergy. Or an allergic reaction can be triggered by taking a drug, eating certain foods, or breathing in dust or animal dander. These diseases include hay fever, food allergies, atopic dermatitis, allergic asthma, and anaphylaxis. Symptoms may include red eyes, an itchy rash, sneezing, a runny nose, shortness of breath, or swelling. Food intolerances and food poisoning are separate conditions.

The immune system is responsible for the identification and destruction of foreign substances that enters the body. Normally the immune system acts as the body's defense against disease-causing microorganisms and substances. However for some people, the immune system mistakes perfectly harmless substances for germs and in response, it releases certain compounds to destroy the perceived "enemy". This results to allergic reaction. Some people may show allergic reaction to one substance, but others may manifest allergic reaction to many substances which we call allergens.

Early exposure to potential allergens may be protective. Treatments for allergies include the avoidance of known allergens and the use of medications such as steroids and antihistamines. In severe reactions injectable adrenaline (epinephrine) is recommended. Allergen immunotherapy, which gradually exposes people to larger and larger amounts of allergen, is useful for some types of allergies such as hay fever and reactions to insect bites. Its use in food allergies is unclear.

Allergies are common: In the developed world, about 20% of people are affected by allergic rhinitis, about 6% of people have at least one food allergy, and about 20% have atopic dermatitis at some point in time. Depending on the country about 1–18% of people have asthma. Anaphylaxis occurs in between 0.05–2% of people. Rates of many allergic diseases appear to be increasing.

Where are Allergens?
Allergens may be inhaled, ingested (eaten or swallowed), applied to the skin, or injected into the body either as a medication or inadvertently by an insect sting. The symptoms and conditions that result depend largely on the route of entry and the type of allergen. The chemical structure of allergens affects the route of exposure. Airborne pollens, for example, tend to have little effect on the skin. They are easily inhaled and will thus cause more nasal and respiratory symptoms with limited skin symptoms. When allergens are swallowed or injected, they may travel to other parts of the body and provoke symptoms that are remote from their point of entry. For example, allergens in foods may prompt the release of mediators in the skin and cause hives.

The specific protein structure is what determines the allergen's characteristics. Cat protein, Fel d 1, from the Felis domesticus (the domesticated cat), is the predominant cat allergen. Each allergen has a unique protein structure leading to its allergic characteristics.

In the Air we Breathe:
Aside from oxygen, the air contains a wide variety of particles, including allergens. The usual diseases that result from airborne allergens are hay fever, asthma, and conjunctivitis. The following allergens can trigger allergic reactions when inhaled by sensitized individuals.

Pollens from trees, grasses, and/or weeds.
Dust mites.
Animal proteins, including dander, skin, and/or urine.
Mold spores.
Insect parts, especially from cockroaches.

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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. 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.

Food Allergy:
A wide variety of foods can cause allergic reactions, but 90% of allergic responses to foods are caused by: cow's milk, soy, eggs, wheat, peanuts, tree nuts, fish, and shellfish. Other food allergies, affecting less than 1 person per 10,000 population, may be considered "rare". The use of hydrolyzed milk baby formula versus standard milk baby formula does not appear to change the risk. Although peanut allergies are notorious for their severity, peanut allergies are not the most common food allergy in adults or children. Severe or life-threatening reactions may be triggered by other allergens, and are more common when combined with asthma.

Peanut allergies are not the most common food allergy in adults or children. Severe or life-threatening reactions may be triggered by other allergens, and are more common when combined with asthma. Peanut allergies can sometimes be outgrown by children. 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-protein allergies are most common in children. Approximately 60% of milk-protein reactions are immunoglobulin E-mediated, with the remaining usually attributable to inflammation of the colon. Some people are unable to tolerate milk from goats or sheep as well as from cows, and many are also unable to tolerate dairy products such as cheese. Roughly 10% of children with a milk allergy will have a reaction to beef. Beef contains small amounts of proteins that are present in greater abundance in cow's milk. Lactose intolerance, a common reaction to milk, is not a form of allergy at all, but rather due to the absence of an enzyme in the digestive tract.

Allergens can be transferred from one food to another through genetic engineering, however genetic modification can also remove allergens. Little research has been done on the natural variation of allergen concentrations in unmodified crops.

Medication Allergy:
About 10% of people report that they are allergic to medications; however, 90% turn out not to be. Serious allergies only occur in about 0.03%. That include: penicillin, allopurinol, sulfa antibiotics, no steroidal anti-inflammatory drugs (NSAIDS, such as aspirin and ibuprofen), muscle relaxants.

Insect Allergy:
Allergy caused by insect bites that belongs to the hymenoptera order of insects. Typically, insects which generate allergic responses are either stinging insects (wasps, bees, hornets and ants) or biting insects (mosquitoes, ticks). Stinging insects inject venom into their victims, whilst biting insects normally introduce anti-coagulants.

Latex Allergy:
Allergy caused by contact to latex materials. Common latex materials that cause allergic reaction are: rubber bands, carpet backing, hospital and dental equipment, rubber gloves, balloons, condoms. 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, 1 in 800 surgical patients (0.125 %) reported 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 48–96 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 a 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, those with latex allergy may also have sensitivities to avocado, kiwifruit, and chestnut. These people often have perioral itching and local urticarial. 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 other plant proteins.

Allergic Conjunctivitis and Allergic Rhinitis:
Allergic reaction is caused by exposure to pollen, house-dust mite, but there are others, including allergens from animals dander such as cats, dogs, and horses. Certain foods, drugs and chemicals can also trigger allergic reaction.

Hay Fever:
Fever, asthma and eczema are all related allergy conditions and the tendency to develop them runs in families. Most people with hay fever have allergic reaction to grass and oilseed rape pollens, which appear from April to August each year. People who are sensitive to pollen from hazel, yew, elm and alder may develop allergy symptoms from January to April, and people allergic to nettles and other weeds can be affected from April to mid-September.

Many individuals have a combination of both seasonal and perennial allergies. Symptoms result from the inflammation of the tissues that line the inside of the nose after exposure to allergens. The ears, sinuses, and throat can also be involved. The most common symptoms include the following: runny or stuffy nose, sneezing, itchy nose, ears, and throat or postnasal drip (throat clearing). In 1819, an English physician, John Bostock, first described hay fever by detailing his own seasonal nasal symptoms, which he called "summer catarrh". The condition was called hay fever because it was thought to be caused by "new hay".

Anaphylaxis:
Anaphylactic shock is a potentially life-threatening allergic reaction that can affect a number of organs at the same time. Allergens that typically lead to anaphylaxis are foods, medications, and venom (bee stings). Environmental allergens rarely lead to anaphylaxis, except anaphylaxis can result from allergy shots (subcutaneous immunotherapy). Some or all of the following symptoms may occur:

Hives itching or flushing present in 80%-90% of cases
Nasal congestion, runny nose, itchy eyes
Swelling of the tongue and/or throat
Abdominal discomfort, nausea, vomiting, diarrhea
Shortness of breath, wheezing, coughing
Low blood pressure, leading to lightheadedness, passing out, or shock.
Anaphylactic shock is an emergent, life-threatening condition that occurs when blood vessels dilate excessively due to an allergic reaction, which causes a significant drop in blood pressure. This can result in inadequate blood flow to the organs in the body.

Exercise Induced Anaphylaxis:
Allergy due to exercise-induced anaphylaxis develops allergic reaction after doing some strenuous exercise.

Toxins Interacting with Proteins:
A 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.

Genetics:
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 those children's allergies are likely to be more severe than those in children of 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.

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.

Hygiene:
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 was that insufficient stimulation of the TH1 arm of the immune system leads to an overactive TH2 arm, which in turn leads 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 they are 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 and epidemiologists 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.

Stress:
Chronic stress can aggravate allergic conditions. This has been attributed to a T helper 2 (TH2)-predominant response driven by suppression of interleukin 12 by both the autonomic nervous system and the hypothalamic–pituitary–adrenal axis. Stress management in highly susceptible individuals may improve symptoms.

Other Environmental Factors:
There are differences between countries in the number of individuals within a population having allergies. 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. Historically, the trees planted in urban areas were predominantly male to prevent litter from seeds and fruits, but the high ratio of male trees causes high pollen counts.

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.

Gut worms 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 humans 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.

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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 increased production of mucus in the lungs, shortness of breath, 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 (urticaria). Food allergies rarely cause respiratory (asthmatic) reactions, or rhinitis. Insect stings, food, antibiotics, and certain medicines may 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 on the rate of severity, anaphylaxis can include skin reactions, bronchoconstriction, swelling, low blood pressure, coma, and death. This type of reaction can be triggered suddenly, or the onset can be delayed. The nature of anaphylaxis is such that the reaction can seem to be subsiding, but may recur throughout a 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 "weal and flare" reaction characteristic of hives and angioedema.

With insect stings a large local reaction may occur (an area of skin redness greater than 10 cm in size). It can last one to two days. This reaction may also occur after immunotherapy.

Affected organ    Common signs and symptoms
Nose    Swelling of the nasal mucosa (allergic rhinitis), runny or stuffy nose, sneezing (hay fever, rhinitis), nasal congestion.
Sinuses    Allergic sinusitis.
Eyes    Redness and itching of the conjunctiva (allergic conjunctivitis, watery), redness under the lids and of the eye overall, swelling of the membranes.
Mouth and throat    Swelling around the mouth and throat, postnasal drip (throat clearing).
Airways    Sneezing, coughing, bronchoconstriction, wheezing and dyspnea, shortness of breath, chest tightness, sometimes outright attacks of asthma, in severe cases the airway constricts due to swelling known as laryngeal edema.
Ears    Feeling of fullness, itchy ears, possibly pain, and impaired hearing due to the lack of eustachian tube drainage.
Skin    Rashes, such as eczema and hives (urticaria), itchy and swelling skin, dry skin associated with significant itching, involvement of the face, the front of elbows, and behind knees, though the rash can occur anywhere, intense itching (typically not painful), raised, red, welts that resolve over hours to a day, swelling (particularly of the lips, face, hands, and feet).
Gastrointestinal tract    Abdominal pain, nausea, bloating, vomiting, diarrhea.
All body    Lowering of blood pressure, light headedness, weakness, and anaphylaxis, collapse or loss of consciousness, in severe cases may cause death.