PPD test

The PPD test is the tuberculin test. It is a skin test based on the principle of type IV allergy, which is used to detect whether the body has been infected with Mycobacterium tuberculosis. Every body that has been infected with Mycobacterium tuberculosis will produce corresponding sensitized lymphocytes and has the ability to recognize Mycobacterium tuberculosis. When a small amount of Mycobacterium tuberculosis or tuberculin is encountered again, the sensitized T lymphocytes are stimulated by the same antigen to release a variety of soluble lymphokines, resulting in increased vascular permeability, macrophages localized, and infiltration. Within about 48 to 72 hours, a positive reaction of swelling and sclerosis occurred locally. If the subject has not been infected with Mycobacterium tuberculosis, there is no local allergy in the injection.