Classification of nerves is modified by the delayed recognition of Mycobacterium leprae

Biopsies of 42 concurrent nerve and skin lesions across the spectrum of leprosy were classified and compared histologically and bacteriologically. Observations were made as follows: a) The bacterial load was higher in nerve than in skin lesions of the same histological classification, and it was higher in nerve than in concurrent skin lesions irrespective of classification, although not at the lepromatous pole. b) There was some discrepancy between the histological classification of nerve and skin lesions in half the cases. Skin classification appeared to represent the general tissue response and, insofar as discrepancies existed, the skin classification was thought to give the better evaluation. Nerve classification was subject to minor variations of a random nature which were thought to be the outcome of local reactions due to the build up of antigen as a result of delayed recognition in an immunologically protected situation. Upgrading or downgrading ensued locally, depending on the level of antigen at the time of its detection. In such cases, the corresponding skin classification was usually BT, which occupied a critical point in the spectrum. A certain autonomy of the response between lesions of skin and nerve suggests an explanation for downgrading reactions. Although Mycobacterium leprae, alone among mycobacteria, has some sort of affinity for Schwann cells, it is the role of the nerves as protected sites which is fundamental to the course of the disease.