Tuberculin is a purified protein derived from tubercle bacilli. Another name for tuberculin is PPD (purified protein derivative). Following infection with M. tuberculosis, a person develops hypersensitivity to tuberculin. Tuberculin injected into the skin of an infected person produces a delayed local reaction after 24-48 hours. This reaction is quantified by measuring the diameter of skin induration (thickening) at the site of the reaction. Various conditions can suppress this reaction. The reaction indicates hypersensitivity. In other words, the reaction only shows that the person has at some time be infected with M. tuberculosis.
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