Suppression of tuberculin skin reactivity by prior tuberculin skin testing.

In two consecutive studies, eighteen and forty-one normal subjects were given a standard tuberculin skin test of 1 tuberculin unit (0.02 mug of PPD). Two days later another skin test was given. Readings of the skin reactions showed that subjects exhibiting strongly positive skin reactions in their primary skin tests showed a significantly smaller indurations in their primary skin tests tended to have equal or larger areas of induration after the secondary skin tests. These observations are discussed in relation to the possible existence of regulatory mechanisms of cell-mediated immune reactions.