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An allergy, also called atopy, is an immediate inflammation reaction caused by environmental exposure to a specific substance. Allergies are a type of hypersensitivity, a term that refers to any undesirable reaction of the immune system to a foreign substance. Immunologists P.H.G. Gell and Robin Coombs classified hypersensitivities into a hierarchy of four types, with allergies designated Type I. An allergic reaction is the hypersensitivity reaction with the fastest onset.
An allergy is a disorder of the immune system. Allergies are specific to particular substances, called allergens, and different people are allergic to different allergens.
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What Happens When an Allergen is Encountered?
An individual can be exposed to an allergen in a number of different ways, for example, through inhalation (dust mite and animal dander), ingestion (food and medications), skin contact (latex and substances in toiletries), or injection (insect stings and medications). If the person has an allergy to that substance, a reaction will be noticeable within seconds or minutes of exposure.
A person may become allergic to an allergen on the first or subsequent exposure if the allergen causes the immune system to produce large amounts of immunoglobulin E (IgE) antibodies specific to the allergen. The IgE then binds to certain white blood cells called mast cells, basophils, and eosinophils, which play an important role in inflammation reactions. These cells are thus primed to react in the presence of the allergen.
When a person is subsequently exposed to the allergen, it binds to the IgE proteins, which cause the bound mast cells, basophils, and eosinophils to release cell contents (degranulation) such as histamine. Mast cells, which are found in connective tissue and mucous membranes, cause inflammation (increased local circulation) by increasing vascular permeability and also cause respiratory problems by constricting smooth muscle in the bronchioles. Basophils are similar to mast cells, but circulate in the blood. Eosinophils are blood cells that carry "chemical weapons" that are toxic to invading organisms, but also to the host tissue.
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Why Do We Have IgE?
IgE antibodies are the cause of all allergic reactions. Since allergy is a non-adaptive and problematic disease, it might seem that IgE is a liability in the body. In fact, IgE is adapted to help the immune system respond to parasites. The mast cells and basophils to which it binds are located in the parts of the body most susceptible to being invaded by parasites. B cells, which secrete antibodies including IgE, are part of the adaptive immune system, the part of the immune system that has a "memory" to allow a fast, specific response to a foreign invader (antigen).
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What Does an Allergic Reaction Look Like?
Allergies have many symptoms, and specific symptoms depend on the allergen, the method and degree of exposure, and individual differences. Allergic diseases include allergic rhinitis (hay fever), asthma, atopic dermatitis, and sometimes the life-threating condition anaphylaxis.