The host defense system in the human newborn: the role of interferon and the natural killer cell.

The human newborn is particularly susceptible to infections in the neonatal period, due in part to the immaturity and naive state of the immune system. The role of interferon and natural killer cells in host defense in this age group is poorly defined. We have studied natural killer cells and the activation of the interferon system in cord blood. Most newborns (75%) had low natural killer cell activity, and this was unrelated to levels of the intracellular interferon-associated enzyme, 2'-5' oligoadenylate synthetase (2-5A synthetase). Newborn cells responded to interferon in vitro with an increase in natural killer activity, suggesting that this low natural killer cell activity is the result of fewer numbers of effector cells rather than a functional immaturity. Circulating serum interferon was detected in greater than 50% of maternal samples tested; however, this did not correlate with 2-5A synthetase levels in maternal or the corresponding cord blood mononuclear cells. Natural killer cell activity was low in the newborn in spite of activation of the interferon system in 39% of these individuals. This may be an important factor in susceptibility to infections in this period.