The study by G. B. Mackaness investigates the cellular resistance to *Listeria monocytogenes* in mice, focusing on the mechanisms of acquired resistance and the role of macrophages. The research highlights that the mouse's native susceptibility to *L. monocytogenes* is due to the bacteria's ability to survive and multiply within host macrophages. During the initial 3 days of infection, bacterial populations in the spleen and liver increase at a constant rate. On the 4th day, the host develops a hypersensitivity reaction to *Listeria* antigens, which coincides with the onset of an antibacterial mechanism. This mechanism involves the transformation of macrophages into cells capable of inactivating ingested *Listeria*. These resistant macrophages first appear in the peritoneal cavity and persist for at least 3 days in vitro. The study also suggests a correlation between the development of delayed-type hypersensitivity and the acquisition of antibacterial properties, indicating that these processes may be linked to a single immunological event. The findings provide insights into the cellular basis of acquired resistance to intracellular pathogens.The study by G. B. Mackaness investigates the cellular resistance to *Listeria monocytogenes* in mice, focusing on the mechanisms of acquired resistance and the role of macrophages. The research highlights that the mouse's native susceptibility to *L. monocytogenes* is due to the bacteria's ability to survive and multiply within host macrophages. During the initial 3 days of infection, bacterial populations in the spleen and liver increase at a constant rate. On the 4th day, the host develops a hypersensitivity reaction to *Listeria* antigens, which coincides with the onset of an antibacterial mechanism. This mechanism involves the transformation of macrophages into cells capable of inactivating ingested *Listeria*. These resistant macrophages first appear in the peritoneal cavity and persist for at least 3 days in vitro. The study also suggests a correlation between the development of delayed-type hypersensitivity and the acquisition of antibacterial properties, indicating that these processes may be linked to a single immunological event. The findings provide insights into the cellular basis of acquired resistance to intracellular pathogens.