The frequencies of total and viable lymphocyte populations are decreased in the PBMCs of ELBW preterm infants compared with FT controls. Blood samples from 29 FT infants (D14 after birth) and 134 ELBW preterm infants (D14, D28, and PMW 36 + 0 after birth) were collected, and phenotypic characterization of the T lymphocyte compartment was performed by multicolor flow cytometry. The figure shows the compiled frequencies of (A) total, (B) viable, (C) CD4+, and (D) CD8+ lymphocytes among the PBMCs. The CD4/CD8 ratio is shown in (E), and the frequency of CD25hi cells within the CD4+ population is shown in (F). Bars show medians with interquartile range. Kruskal-Wallis test with Dunn multiple comparison tests was used for group comparisons. PCA comparing the T cell phenotype at D14 of FT neonates (black) with the ELBW neonates born week 23–27 (in color) is shown in (G). The correlation between the percentage of lymphocytes at D14 and birth weight is shown in (H) for the ELBW neonates.