Lymphocyte subpopulations in the blood of newborn infants.
Assays of lymphocyte subpopulations and function have been applied to cells from the cord blood of twenty-four infants. The results are compared with those obtained in healthy adults. T cells, assayed by spontaneous rosette formation with sheep red blood cells (E rosettes) were present in lower proportion in cord (53%) than in adult blood (65%). There was a higher proportion of lymphocytes bearing stainable immunoglobulin in cord (32%) than in adult blood (22%). From the blood lymphocyte counts it was calculated that both T and B lymphocytes are present in greater numbers in the newborn infants' blood than in adults. Comparison of DNA synthesis showed that cord blood leucocytes had a higher spontaneous rate, but there were only minor differences in the lymphocyte mitotic response to phytohaemagglutinin (PHA). The response of cord blood lymphocytes was slightly lower to a submaximal stimulus and higher to a maximal stimulus. There was a correlation between the submaximal response and the proportion of E rosetting cells. 
 
The most striking differences between infant and adult blood lymphocytes were in their cytotoxic activity against homologous target cells (Chang cells). Antibody-dependent cytotoxicity (K-cell activity) was readily detected using cord blood leucocytes, though it was lower than that of adult cells. PHA-induced cytotoxicity was very low in all cord blood samples, and in many cases was almost unmeasurable. This dissociation between the two types of cytotoxic activity is consistent with other evidence that they may be mediated by different cell types. 
 
The assays were also applied to blood samples taken from five mothers of tested infants immediately after delivery. While some differences from normal adults were found with the mothers' lymphocytes they did not mirror those of the cord blood samples. This suggests that the pattern found for cord blood lymphocytes is not due to maternal factors crossing the placenta.