Stochastic stimulation of proliferating T cells leads to intraclonal functional diversification. By establishing immunological synapses with dendritic cells (DCs), naïve T cells (green) achieve stimulation and become committed to proliferate in response to autocrine or paracrine IL-2. T-cell receptor stimulation is sustained by serial encounters with DCs and, in the presence of polarizing cytokines (IL-12 and IL-4, not shown), drives T-cell differentiation to Th1 or Th2 effector cells that have lost CCR7 expression (red). T cells receiving a shorter stimulation do not acquire effector function and do retain lymph-node homing capacity (yellow). An excessive stimulation leads to activation-induced cell death (black).