Age-Related Development of Human Natural Killer Cell Activity

To the Editor: Because of their ability to lyse tumor and virally infected cells spontaneously in vitro, natural killer cells may represent a first line of defense against malignancy and pathogenic invasion. 1 It is known that natural killer activity is acquired sometime after birth in both animals 1 2 3 and human beings. 4 , 5 However, there have been no reports of the actual time it takes for natural killer activity to become fully developed in human beings. While investigating this activity in persons with various disorders, we observed that full natural killer cell activity was not attained until about four years of age. . . .