NK cells and their receptors during viral infections.

Increasing evidence indicates the importance of human natural killer (NK) cells in the immune response against certain viral infections. In the present article, we summarize information on NK cell responses against several viruses and on the nature of NK cell receptor-ligand interactions involved in these responses. Recent studies indicate that NK cells display functional features that are normally attributed exclusively to cells of the adaptive immune system. In this context, experiments both in mice and humans suggest the existence of long-lived NK cells that expand during viral infections and retain a 'memory' of previous exposure to a specific antigen. However, further studies are necessary to better define the characteristics of these long-lived NK cell populations and their role in viral infections.