It has been demonstrated that normal human newborns had significantly lower percentages of CD8+, CD11b+, CD 16+ and CD 56+, compared to adults, while no differences had been noted for the absolute numbers of these cells. Both the proportions and absolute numbers of CD 5 7+ lymphocytes and cytotoxic T cells that mediate non-MHC-restricted cytotoxicity (CD3+CD56+) were markedly reduced in neonatal blood. Mean proportions but not absolute numbers of precursors and effectors of allocytotoxic T cells (CD8+CD11b––) significantly decreased in human neonates. Dual-labeling experiments demonstrated that newborns’ natural killer cells had a phenotype different from that of adults. In newborns, there were much higher levels of CD1 1b+ cells, much lower levels of CD57+ cells and similar levels of CD8+ lymphocytes among CD 16+ lymphocytes. It seems that a phenotype immaturity of natural killer cells may contribute to the diminished cytotoxic potential of neonatal lymphocytes.
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