PIP: In this review various aspects of tuberculo-immunity and of the pathogenesis and diagnosis of tuberculosis (TB) are examined in the light of current immunological knowledge. Successful resistance in TB is not easily predicted by the presence or absence of an immune response, but it is a balance between the various types of immune responses of the host and the strength of the infecting bacterial strain for that individual host. Primary infection is usually acquired through the inhalation of infected droplets; the probability of the development of disease among TB-positive individuals varies from 30/100,000 in Denmark to 600/100,000 in some Eskimo populations. Even if TB were eradicated, however, sporadic cases would continue to occur since for TB acquired from the environment control would necessitate lowering the rate of infection in bird and animal groups. 4 groups of TB patients include: 1) those in which cell-mediated activity is fully active, 2) those in which cell-mediated immunity is not detected, 3) an intermediate group leaning more towards group 1, and 4) an intermediate group leaning towards group 2. The cells involved in the outcome of a TB infection, macrophages and lymphocytes are discussed individually. The thick cell wall of the Mycobacterium tuberculosis resists destruction by enzymes, antibodies, and other factors and requires a mechanism that will activate macrophages and histocytes to breach this cell wall and lead to cell death. A vaccine against TB should be long lasting, such as BCG, although BCG has not been found to be effective in all populations. The degree of immune response to TB will depend on: 1) the degree of sensitivity the mycobacterial species is capable of inducing, and 2) how much of this sensitivity is cross-reactive with M. tuberculosis. High effectiveness of BCG immunization has been related to a low endemicity of non-TB mycobacteria; it should be administered early in life in countries with a high prevalence of TB. The pathogenesis and pathology of TB is discussed as are immunological tests such as the tuberculin tests which indicates TB sensitivity, in vitro tests for cellular hypersensitivity, and dual skin tests. Future research should focus on the BCG vaccine, better in vitro tests, and standardization of reagents.