Fig. 1.

In young age (left panel), the thymus generates a large number of naïve CD4⁺ T cells every day. These cells reach the periphery where they form the majority of the CD4⁺ T cell pool, with fewer memory and regulatory CD4⁺ T cells. In old age (right panel), thymic involution leads to a much reduced output of naïve CD4⁺ T cells and a relatively increased output of regulatory T cells. Very few naïve CD4⁺ T cells therefore reach the periphery where memory CD4⁺ T cells, generated through antigen encounters throughout life and other factors (see text), accumulate.