pH changes in the dermis during the course of the tuberculin skin test

The response of six healthy young adults to tuberculin skin testing was studied. Five subjects developed a typical delayed-type hypersensitivity reaction to PPD with a local rise in skin temperature, and the sixth showed a less intense response; a considerable increase in blood flow velocity was seen in all reactions. All subjects showed a fall in pH in the dermis during the course of the reaction: in four subjects the pH minimum occurred at the time when the changes of erythema and induration were most prominent, in one subject the pH fall preceded the maximal clinical changes, and in the remaining subject a substantial fall in pH occurred with only transient erythema. It was concluded that the local tissue acidosis had resulted from the greatly increased metabolic demand of the lymphocytes and monocytes attracted into the dermis as part of the type IV delayed-type hypersensitivity reaction, and that the concurrent reactive hyperaemia was insufficient to clear the acidic metabolic products of the greatly increased cell population.