Tuberculin is the name given to extracts of Mycobacterium tuberculosis, M. Bovis, or M. avium, used in skin testing in animals and humans to identify a tuberculosis infection. Several types of tuberculin have been used for this, of which purified protein derivative is the most important. PPD is a poorly defined complex antigen mixture. Tests based upon PPD are relatively unspecific since many of it's proteins are found in different...