An Id reaction to Mycobacterium leprae: first documented case.

We present a case of generalized sarcoidal dermatitis in a patient with tuberculoid leprosy. After careful consideration of the reactional states in leprosy and recognizing that id reactions occur in other mycobacterial infections, we concluded that the patient's eruption was likely an id reaction to his Mycobacterium leprae infection. It was most reminiscent of the tuberculids: lichenoid tuberculid and lichen scrofulosorum. However, its histomorphologic appearance was distinctive, showing elongated granulomata along with foci of incipient perineural granuloma formation, similar to the histomorphologic appearance of tuberculoid leprosy. A pathogenetic mechanism for this unique eruption is reviewed briefly.