Immunophenotyping of blood lymphocyte subpopulations is an important tool in the diagnosis of immunological and hematological diseases. Pediatric age-matched reference values have been determined for the major lymphocyte populations, but reliable reference values for the more recently described T-lymphocyte subpopulations, like different types of memory T-lymphocytes, recent thymic emigrants, regulatory T-cells and CXCR5+ helper-T-lymphocytes, are not sufficiently available yet. We determined reference values for the absolute and relative sizes of T-lymphocyte subpopulations in healthy children using the lysed whole blood method, which is most often used in diagnostic procedures. When the absolute numbers of some or all T-lymphocyte subpopulations fall outside these reference ranges, this may indicate disease.
The reference values show the course of T-lymphocyte development in healthy children. Absolute T-lymphocyte numbers increase 1.4-fold during the first months of life and after 9-15 months they decrease threefold to adult values; this is mainly caused by the expansion of recent thymic emigrants and naive cells. Helper and cytotoxic T-lymphocytes show the same pattern. Regulatory T-cells increase in the first five months of life and then gradually decrease to adult values, although the absolute numbers remain small. The relative number of CXCR5+ cells within the CD4+CD45RO+ T lymphocytes increases during the first six months of life and then remains more or less stable around 20%.