This article makes no attempt to review the literature in a complete fashion, but relies on articles which do so. References to such articles are printed in italic type. In this review, cellular hypersensitivity, cellular allergy, and delayed hypersensitivity are used interchangeably; and macrophage activation is used synonymously with macrophage maturation and macrophage differentiation. Although macrophages frequently proliferate during the activating process, activation can occur without proliferation. These two terms are therefore used independently. with current knowledge. These assumptions are then used to explain the known facts concerning the pathogenesis of tuberculosis. Finally, the relative importance of local and systemic aspects of cellular immunity is discussed. Nature and Effects of Cellular Hypersensitivity In tuberculosis, cellular hypersensitivity, the delayed type of allergy, may be defined as an immunological state in which lymphocytes and macrophages are directly or indirectly sensitive to tuberculin. As a result of this sensitivity, tuberculin in the appropriate low concentration will activate macrophages, i.e., cause the production of new lysosomes and other organelles as well 85