Skin changes in the tuberculin test

This review describes the recent advances in knowledge of the nature and range of physiological changes that occur in the skin at the site of a positive tuberculin reaction. The infiltration of T-cells and monocyte/macrophages shows a marked compartmentalisation suggesting that the.functions of particular cell types depend on their localisation. The extent of cutaneous oedema (detectable as induration) is not closely related to other features of the reaction or to systemic indicators of cell mediated immunity. The intensity of hyperaemia is maximal at the centre of the reaction and is correlated in most cases with the density of cellular infiltration in the dermis suggesting a functional coordination. Despite this correlation between cell numbers and velocity of blood flow, the reaction normally shows hypoxia, hypercapnia and local acidosis, but this metabolic modification may not be a wholly disadvantageous effect since these conditions appear to facilitate the growth and metabolism of activated lymphocytes and macrophages. In very strong reactions, there is central relative slowing of the circulation and this may lead to necrosis in extreme cases.

There are however a minority of cases where cell infiltration occurs but induration is not palpable: this situation has been named pseudoanergy, and its pathogenesis has not yet been established. The occurrence of pseudoanergy must throw some doubt on the conventional criteria for positivity in the reading of tuberculin skin tests (induration >5 mm) and this may have relevance to future strategies for assessment of new vaccines.

The human tuberculin reaction should prove a valuable model for coordinating knowledge of the cellular and molecular mechanisms induced by mycobacterial immunity with the pathogenesis of tissue reactions in clinical tuberculosis. This should lead to the rational development of therapy for limiting inflammatory and scarring damage in antibiotic treated mycobacterial disease.