Figure 2. There are two dominant competing models of T cell differentiation.

Set of panels shown on the left side depicts the ‘off-on-off’ model, in which all memory T cells are derived from cells that were once effector cells. Shown in the right set of panels is the ‘developmental’ model whereby memory cells arise directly out of naïve precursors without going through an effector stage. These distinct models lead to different predictions, which are explained in detail in the text.