Hypersensitivity (immunology) - Definition

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Hypersensitivity relating to an immunological process is an exaggerated, even dangerous, reaction to an intrusion, no matter how small, in the body, that would not have provoked a reaction in those that are not hypersensitive. This immune response can be generated by a toxin, virus, bacteria, or allergen. Certain reactions are called allergies, which can be severe enough to be fatal, such as cases of anaphylactic shock. Hypersensitivity initially presents a sensitizing phase, followed by a latency phase. During the lesion phase, in other words on second contact with the allergen, the symptoms appear.