Allergy

An allergy is an exaggerated immune response where the body mistakenly identifies an ordinarily harmless allergen as a threat. Allergic reactions give rise to allergic diseases such as hay fever, allergic conjunctivitis, allergic asthma, atopic dermatitis, food allergies, and anaphylaxis. Symptoms of allergic diseases may include red eyes, an itchy rash, sne

An allergy is an exaggerated immune response where the body mistakenly identifies an ordinarily harmless allergen as a threat.[11][12][13][14] Allergic reactions give rise to allergic diseases such as hay fever, allergic conjunctivitis, allergic asthma, atopic dermatitis, food allergies, and anaphylaxis.[1] Symptoms of allergic diseases may include red eyes, an itchy rash, sneezing, coughing, a runny nose, shortness of breath, or swelling.[15][3][4]

Common allergens include pollen, certain foods, metals, insect stings, medications, and materials, such as latex.[11][2] The development of allergies is due to genetic and environmental factors.[2] The mechanism of allergic reactions involves immunoglobulin E antibodies (IgE) binding to an allergen and then to a receptor on mast cells or basophils, where they trigger the release of inflammatory chemicals such as histamine.[16] Diagnosis is typically based on a person's medical history.[3] Further testing of the skin or blood may be useful in certain cases.[3] Positive tests, however, may not necessarily mean there is a significant allergy to the substance in question.[17]

Early exposure of children to potential allergens may be protective.[5] Treatments for allergies include avoidance of known allergens and the use of medications such as steroids and antihistamines.[6] In severe reactions, injectable adrenaline (epinephrine) is recommended.[7] 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.[6] Its use in food allergies is unclear.[6]

Allergies are common.[10] In the developed world, about 20% of people are affected by allergic rhinitis,[18] food allergy affects 10% of adults and 8% of children,[19] and about 20% have or have had atopic dermatitis at some point in time.[20] Depending on the country, about 1–18% of people have asthma.[21][22] Anaphylaxis occurs in between 0.05–2% of people.[23] Rates of many allergic diseases appear to be increasing.[7][24][25] The word "allergy" was first used by Clemens von Pirquet in 1906.[2]

Signs and symptoms

Many allergens, such as dust or pollen, are airborne particles. In these cases, symptoms arise in areas exposed to air, such as the eyes, nose, and lungs.[26] For instance, allergic rhinitis, also known as hay fever, irritates the nose, sneezing, itching, and redness of the eyes.[27] Inhaled allergens can also lead to increased production of mucus in the lungs, shortness of breath, coughing, and wheezing.[28] Allergic rhinitis can simultaneously occur with other allergic responses such as asthma.[29]

Aside from these ambient allergens, allergic reactions can result from foods, insect stings, and reactions to medications like aspirin and antibiotics such as penicillin.[26][30] Symptoms of food allergy include abdominal pain, bloating, vomiting, diarrhea, itchy skin, and hives. Food allergies rarely cause respiratory (asthmatic) reactions, or rhinitis.[31]

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.[32][33][34] Depending on the severity, anaphylaxis can include skin reactions, bronchoconstriction, swelling, low blood pressure, coma, and death. This type of reaction can be triggered suddenly or have a delayed onset. The anaphylactic reaction may seem to subside, but may recur over time.[34]

Skin

Substances that come into contact with the skin, such as latex, are also common causes of allergic reactions, known as contact dermatitis or eczema.[35] 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.[36]

With insect stings, a large local reaction may occur as an area of skin redness greater than 10 cm in size that can last one to two days.[37] This reaction may also occur after immunotherapy.[38]

The skin forms an effective barrier to the entry of most allergens, but this barrier cannot withstand everything. For example, an insect sting can breach the barrier and inject allergen into the affected spot. When an allergen enters the epidermis or dermis, it triggers a localized allergic reaction which activates the mast cells in the skin resulting in an immediate increase in permeability of blood vessels, leading to fluid leakage and swelling in the affected area.[39] Mast-cell activation also stimulates a skin lesion called the wheal-and-flare reaction.[40] Then the release of chemicals from local nerve endings by a nerve axon reflex causes the widening of surrounding cutaneous blood vessels, which causes redness of the surrounding skin.[40]

As part of the allergic response, the body has developed a secondary response that, in some individuals, causes a more widespread and sustained skin swelling.[39] This usually occurs about 8 hours after the allergen originally comes in contact with the skin. When an allergen is ingested, a dispersed form of wheal-and-flare reaction, known as urticaria or hives, will appear when the allergen enters the bloodstream and eventually reaches the skin.[39][41] The way the skin reacts to different allergens and allows to test for exiting allergies by injecting a very small amount of an allergen into the skin, which cause skin reaction in the site of injection.[39] Even though these injections are very small and local, they still pose the risk of causing systemic anaphylaxis.[39]

Airways

Substances that come into contact with the airways may trigger allergic reactions in the nose, sinuses, throat and lungs. Allergic rhinitis, often referred to as hay fever, is one of the most common airway allergies and is characterized by sneezing, nasal congestion, rhinorrhea and itching of the nose and eyes.[42] Airborne allergens commonly associated with allergic rhinitis include pollen, house dust mites, animal dander and mold spores.[43]

Allergic reactions involving the lower airways may contribute to allergic asthma, in which exposure to allergens can lead to bronchoconstriction, wheezing, coughing and shortness of breath.[44] Common triggers include pollen, dust mites, animal allergens and occupational exposures such as flour dust or chemical sensitizers.[45]

Some allergic airway reactions may occur rapidly after exposure, while others develop over several hours. Severe reactions involving airway swelling may impair breathing and constitute a medical emergency.[46]

Cause

Risk factors for allergies can be placed in two broad categories: host and environmental factors.[47] Host factors include heredity, sex, race, and age, with heredity being by far the most significant. However, there has been a recent increase 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.[48]

Dust mites

Dust mite allergy, also known as house dust allergy, is a sensitization and allergic reaction to the droppings of house dust mites. The allergy is common[49] and can trigger allergic reactions such as asthma, eczema, or itching.[50]

The mite's gut contains potent digestive enzymes (notably peptidase 1) that persist in their faeces and are major inducers of allergic reactions such as wheezing. The mite's exoskeleton can also contribute to allergic reactions.[51] Unlike scabies mites or skin follicle mites, house dust mites do not burrow under the skin and are not parasitic.[52]

Dust mite-proof encasements to mattress, pillow, and duvet prevent chronic contact with allergens.[53]

Foods

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.[54] Other food allergies, affecting less than 1 person per 10,000 population, may be considered "rare".[55] The most common food allergy in the US population is a sensitivity to crustacea.[55] Although peanut allergies are notorious for their severity, peanut allergies are not the most common food allergy in adults or children. Other allergens may trigger severe or life-threatening reactions and are more common when combined with asthma.[54]

Rates of allergies differ between adults and children. Children can sometimes outgrow peanut allergies. Egg allergies affect one to two percent of children but are outgrown by about two-thirds of children by the age of 5.[56] The sensitivity is usually to proteins in the white, rather than the yolk.[57]

Milk-protein allergies—distinct from lactose intolerance—are most common in children.[58] Approximately 60% of milk-protein reactions are immunoglobulin E–mediated, with the remaining usually attributable to inflammation of the colon.[59] 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.[60] Lactose intolerance, a common reaction to milk, is not a form of allergy at all, but due to the absence of an enzyme in the digestive tract.[61]

Those with tree nut allergies may be allergic to one or many tree nuts, including pecans, pistachios, and walnuts.[57] In addition, seeds, including sesame seeds and poppy seeds, contain oils in which protein is present, which may elicit an allergic reaction.[57]

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.[62][63]