Natural killer cells in the late decades of human life.

We examined the lymphocyte subsets and indices of natural killer (NK) cell activity (lytic unit (LU), index of absolute NK cell activity in vivo (ALU), and NK cell activity on a per-cell basis (PCNK)) in 82 people (age, 30-99 years) who were immunologically normal. Although the number of NK cells was maintained throughout the examined age range, the ALU and PCNK values correlated negatively with age. We then examined whether any of the various immunologic parameters, including the function and cell counts of NK cells, T cells, and neutrophils, related to past infectious episodes and death in the follow-up period in 44 elderly subjects (age, 63-98 years). Only low ALU and PCNK values correlated with a past history of severe infection, while low LU, ALU, and PCNK values were the only parameters which correlated with death due to infection during the follow-up period. We propose that human NK cells do not escape the aging process and that a low NK cell function relates to the development of severe infections, which may be fatal, in elderly subjects.