The relevance to protection of three forms of delayed skin-test response evoked by m. leprae and other mycobacteria in mice. Correlation with the classical work in the guinea-pig.

The controversy surrounding the protective role of 'delayed hypersensitivity' in tuberculous guinea-pigs has never been resolved. This controversy has arisen because the term 'delayed hypersensitivity' is used indiscriminately to describe both a type of necrotic skin-test reactivity which does not appear until 4-6 weeks after infection, and also non-necrotic reactions which can be elicited within a few days. Responses closely analogous to both have been characterized in mice immunized with mycobacteria. Simple criteria are described which allow these responses to be distinguished from one another, and from the Jones-Mote phenomenon. The relevance of each type to protection, susceptibility and immunopathology in leprosy, tuberculosis, leishmaniasis and listeriosis is discussed.