Natural killer cells remember: an evolutionary bridge between innate and adaptive immunity?

Abstract

Since their discovery three decades ago, NK cells have been classified as cells of the innate immune system. NK cells were shown to respond rapidly and non-specifically to infection, and were thought to act as a functional "bridge" to sustain the early innate immune response until the later adaptive immune responses could be mounted. In light of new findings showing how NK cells possess nearly all of the features of adaptive immunity including memory, we propose the placement of NK cells as an "evolutionary bridge" between innate and adaptive immunity.

Figure 1

Traditional and revised models of innate and adaptive immunity. Graphs show the immune response against primary and secondary infections. The response of myeloid cells (neutrophils, macrophages, and dendritic cells) is depicted by the green lines. The NK cell response is shown in purple, and the response of B and T cells is shown in blue. The magnitude of the response refers to the qualitative nature of the immune response (i.e. efficacy (protective capability) and effector cytokine production) rather than the absolute number of cells.