Although natural killer (NK) cells are defined as a component of the innate immune system, they exhibit certain features generally considered characteristic of the adaptive immune system. NK cells also participate directly in adaptive immune responses, mainly by interacting with dendritic cells. Such interactions can positively or negatively regulate dendritic cell activity. Reciprocally, dendritic cells regulate NK cell function. In addition, 'NK receptors' are frequently expressed by T cells and can directly regulate the functions of these cells. In these distinct ways, NK cells and their receptors influence the adaptive immune response.