Estimating total body naive CD4⁺ T cells as a function of age. (A) Body weight as a function of age, from Kuczmarski et al.¹⁰  (B) Blood volume as a function of age, estimated from panel A and the regression relation between weight and blood volume in Linderkamp et al¹¹  ( Appendix). (C) Mean naive CD4⁺ T-cell density in blood with age, using the relation in Huenecke et al⁷  ( Appendix). (D) Estimated total body naive CD4⁺ T-cell numbers, using the data in panels B and C. Panels A, B, and D show the mean, 2.5 and 97.5 percentiles at the population level; panel C shows the mean and its associated 95% confidence interval.