The immune response is initiated in organized lymphoid tissues where antigen-loaded dendritic cells (DCs) encounter antigen-specific T cells. DCs function as packets of information that must be decoded by the T cell before an appropriate immune response can be mounted. We discuss how the dynamics of DC–T cell encounter and the mechanism of T cell differentiation make the decoding of this information stochastic rather than determinate. This results in the generation of both terminally differentiated effector cells and intermediates that play distinctive roles in protection, immunoregulation, and immunological memory.