The time-division based program (5)–(6). APCs and naïve cells are modeled in the same way as in the cell division-based program. (1) Naïve T cells, T0, are stimulated by mature APCs and enter the pool of cells undergoing a minimal developmental program, denoted by T1/2. (2) After σ units of time (the duration of the minimal program) cells enter the effector state, T1, after having divided m times. (3) Cells live in the effector state for τ units of time before committing apoptosis. (4) As an ongoing step, effector cells repeatedly get stimulated by mature APCs and enter the dividing population, T2, where they spend ρ units of time (the duration of one cell division) before returning to the effector state after having divided once.