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Flow cytometric analysis of major lymphocyte populations and their subsets reveals age-related changes in the human cellular immune system.Immunophenotypic markers were evaluated in 136 healthy pediatric subjects divided into groups of newborn infants (cord blood), children aged 1 to 2 years, 2 to 5 years, and 6 to 15 years.The percentage of T cells increased gradually with age and the evolution of the percentage of B and NK cells was found to be variable. The percentage of CD4+ cells remains relatively unchanged from infancy to adolescence, but the percentage of CD8+ T cells was lowest at birth and reached maximal levels in the one to two year-old period. The percentage of naïve T cells declined with time, but the percentage of memory T cells increased with age. Similar trends were seen in T-cell receptor αβ- and γδ-bearing T cells. The percentage of CD11b+CD8+ T cells increased gradually from birth and reached maximal levels from 6 to 15 years old. The expression of the activation markers CD25 and HLA-DR on CD4+ T cells increased with age. The percentage of CD16+CD56− NK cells declined with age, but the evolution of the percentage of CD16−CD56+ NK cells was variable. The fraction of B cells that expressed CD5 was high at birth (72.9%) and was highest in one to two year olds (73.1%), then declined steadily over time. The CD23 antigen was expressed on 41.9% of B cells at birth and 68.6% during the first to second year, then declined steadily with age.These data may serve as a reference range for studies of Chinese pediatric subjects.