When acquired resistance cannot be demonstrated to depend on antibody, it is legitimate to ask whether specifically reactive lymphyocytes are at work as the mediators of immunity, hence to consider the possibility that we are dealing with an example of cell-mediated immunity (CMI). This, however, is not the only sense in which the term “cellular immunity” is applied. It is also used to describe a defense mechanism in which the phagocytic cells of the immune host display abnormally high levels of antimicrobial activity. In this circumstance the ultimate effector cell is an altered macrophage. These two concepts of cellular immunity are related but they are not identical. Although CMI often works through its capacity to potentiate the antimicrobial capacity of host macrophages, other effector mechanisms may also operate in CMI.