Allergic reaction

n. hypersensitivity reaction, excessive sensitivity to a specific allergen, excessive sensitivity to certain substances (i.e. animal hair or pollen) which often causes the development of respiratory problems or other symptoms

Allergy is an abnormal reaction to a substance foreign to the body that is acquired, predictable and rapid. In the strict sense of its meaning, it is the first (type 1) of five forms of hypersensitivity described by Gell and Coombs in their 1963 classification. However, by extension, the term "allergy" is often used for other abnormal reactions to substances. The term was coined by the Viennese pediatrician Clemens von Pirquet in 1906 after noting that some of his patients were hypersensitive to normally innocuous entities such as dust, pollen, or certain foods. Pirquet called this phenomenon "allergy", from the Greek words allos meaning "other" and ergon meaning "work".
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Allergic reaction n. A local or generalized reaction of an organism to internal or external contact with a specific allergen to which the organism [more]Allergic reaction - Community and Resources

Noun
1. hypersensitivity reaction to a particular allergen; symptoms can vary greatly in intensity
(synonym) allergy
(hypernym) hypersensitivity reaction
(hyponym) food allergy

A reaction that occurs when the immune system attacks a usually harmless substance (an allergen) that gains access to the body. The immune system calls upon a protective substance called immunoglobulin E (IgE) to fight these invading allergic substances ( allergens). Even though everyone has some IgE, an allergic person has an unusually large army of these IgE defenders -in fact, too many for their own good. This army of IgE antibodies attacks and engages the invading army of allergic substances of allergens. As is often the case in war, innocent bystanders are affected by this battle. These innocent bystanders are special cells called mast cells. These cells are frequently injured during the warring of the IgE antibodies and the allergic substances. When a mast cell is injured, it releases a variety of strong chemicals including histamine into the tissues and blood that frequently cause allergic reactions. These chemicals are very irritating and cause itching, swelling, and fluid leaking from cells. These allergic chemicals can cause muscle spasm and can lead to lung and throat tightening as is found in asthma and loss of voice.