Recirculating virgin CD4⁺ T cells spend their life migrating between the T zones of secondary lymphoid tissues where they screen the surface of interdigitating dendritic cells. T‐cell priming starts when processed of interdigitating dendritic cells. T‐cell priming starts when processed peptides or superantigen associated with class II MHC molecules are recognised. Those primed T cells that remain within the lymphoid tissue move of the outer T zone, where they interact with B cells that have taken up and processed antigen. Cognate interaction between these cells initiates immunoglobulin (Ig) class swith‐recombination and proliferation of both B and T cells; much of this growth occurs outside the T zones. B cells both B and T cells; much of this growth occurs outside the T zones. B cells migrate to follicles. Where they form germinal centres, and to extrafollicular sites of B‐cell growth, where they differentiate into mainly short‐lived plasma cells. T cells do not move to the extrafollicular foci but to the follicles; there they proliferate and are subsequently involved in the selection of B cell that have mutated their Ig variable‐region genes. During primary antibody responses T‐cell proliferation in follicles produces many times the peak number of T cell found in that site; a substantial proportion of the CD4⁺ memory T‐cell pool may originate from growth in follicles.