HYPERSENSITIVITY REACTION

Abstract

The term hypersensitivity arose from the clinical definition of immunity as “sensitivity” which is based on the observation that an individual who has been exposed to an antigen exhibits a detectable reaction or is “sensitive” to subsequent encounters with that antigen. Thus, hypersensitivity can be defined as a state of increased reactivity to a foreign agent usually acquired by a prior exposure to the same or a chemically related substance. Hypersensitivity reactions require a presensitized state of host. Sensitization, or induction of the hypersensitive state, requires one or more than one exposure to the antigen, followed by a latent period. After the latent period, a reaction is elicited by another exposure to the antigen. The nature of hypersensitivity acquired is determined by the chemical composition of the allergen, the sensitizing route, the physiology and the anatomy of the host and other factors. The original concept of allergy included all immune-mediated diseases and the term allergy was interchangeable with the term “hypersensitivity.” The term allergy was originally defined by Clemens Von Pirquet as “an altered capacity of the body to react to a foreign substance, " which was an extremely broad definition that included all immunological reactions. Allergy is now defined in a much more restricted manner as “disease following a response by the immune system to an otherwise innocuous antigen.” Allergy is one of a class of immune system responses that are termed hypersensitivity reactions. These are harmful immune responses that produce tissue injury and may cause serious disease. Almost every individual is allergic to some medium or the other in his/her lifetime. No human body is of the same composition and hence what could be allergic to someone can be harmless to another person.