Stable isotope labeling is a generally applicable method of quantifying cell dynamics. Its
advent has opened up the way for the quantitative study of T cells in humans. However, the
literature is confusing as estimates vary by orders of magnitude between studies. In this
short review we aim to explain the reasons for the discrepancies in estimates, clarify which
estimates have been superseded and why and highlight the current best estimates. We
focus on stable isotope labeling of T cell subsets in healthy humans.