Human newborns are deficient in natural killer activity.

The peripheral blood natural killer (NK) activity of newborns was found to be significantly less than that of adults. In mixing experiments newborn cells inhibited adult NK activity in only one of nine instances. Interferon treatment in vitro increased newborn NK activity to an even greater degree than adult NK activity. These findings imply that diminished newborn NK activity is due not to inhibitory cells or lack of pre-NK cells but rather to deficient in vivo activation of pre-NK cells. This deficiency may be a major factor in the increased susceptibility of newborns to certain virus infections.