In human peripheral blood, five NK cell subpopulations can be defined on the basis of the relative expression of the markers CD16 and CD56: (1) CD56bright CD16- (50-70% of CD56bright), (2) CD56bright CD16dim (30-50% of CD56bright), (3) CD56dim CD16-, (4) CD56dim CD16bright, and (5) CD56- CD16bright. In healthy individuals, populations (3) and (5) are numerically in the minority.