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 classificationoun 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".