The histological course of reactions in borderline leprosy and their outcome.

The histological course and the outcome of reactions was followed in 12 patients who were all in the borderline (BT-BL) zone of the spectrum at some time during the reaction. In eight patients the reaction proved to be of the upgrading or reversal type with a shift in classification toward the tuberculoid pole. In two patients the reaction was of the opposite downgrading type, and in two there was no significant change in classification as a result of the reaction. Early reactions were characterized by edema and an increase in the number of lymphocytes which was more marked in upgrading than downgrading reactions, but at this stage the outcome could not be reliably predicted. In the acute stage necrosis was apparent in severe cases and was followed by giant cell formation and the evolution of the granuloma cells depending on the direction of the shift: organized clusters of mature epithelioid cells in upgrading, macrophages in downgrading. At all stages the collagen of the dermis might be involved in varying degree, which in extreme cases included fibrinoid necrosis and scarring in the late stage. The histology is compared to that of tuberculin reactions. All cases were bacteriologically positive at the onset of the reaction. The results were consistent with the view that these reactions are delayed type hypersensitivity phenomena associated with the unmasking of bacterial antigen in nerve or other protected sites, or with an increase of hypersensitivity as a result of treatment. They demonstrate the evolution of a granuloma in mycobacterial disease.