The study investigates the adaptive immune response of Natural Killer (NK) cells in a mouse model of cytomegalovirus (MCMV) infection. Unlike traditional views, NK cells, which are typically classified as part of the innate immune system, exhibit characteristics similar to cytotoxic T lymphocytes (T cells) in their ability to undergo proliferation and generate memory cells. Specifically, NK cells bearing the virus-specific Ly49H receptor show significant proliferation (100-fold in the spleen and 1000-fold in the liver) following MCMV infection. These "memory" NK cells persist in lymphoid and non-lymphoid organs for several months, degranulate, and produce cytokines upon reactivation. Adoptive transfer of these NK cells into naive animals followed by viral challenge results in a robust secondary expansion and protective immunity. This research highlights the previously underappreciated adaptive immune properties of NK cells, suggesting that they may have evolved as a precursor to T cells or co-evolved with them, potentially bridging the gap between innate and adaptive immunity.The study investigates the adaptive immune response of Natural Killer (NK) cells in a mouse model of cytomegalovirus (MCMV) infection. Unlike traditional views, NK cells, which are typically classified as part of the innate immune system, exhibit characteristics similar to cytotoxic T lymphocytes (T cells) in their ability to undergo proliferation and generate memory cells. Specifically, NK cells bearing the virus-specific Ly49H receptor show significant proliferation (100-fold in the spleen and 1000-fold in the liver) following MCMV infection. These "memory" NK cells persist in lymphoid and non-lymphoid organs for several months, degranulate, and produce cytokines upon reactivation. Adoptive transfer of these NK cells into naive animals followed by viral challenge results in a robust secondary expansion and protective immunity. This research highlights the previously underappreciated adaptive immune properties of NK cells, suggesting that they may have evolved as a precursor to T cells or co-evolved with them, potentially bridging the gap between innate and adaptive immunity.