Germinal center (GC) B cells undergo affinity selection, which depends on interactions with CD4⁺ follicular helper T cells (T_FH cells). We found that T_FH cells progressed through transcriptionally and functionally distinct stages and provided differential signals for GC regulation. They initially localized proximally to mutating B cells, secreted interleukin 21 (IL-21), induced expression of the transcription factor Bcl-6 and selected high-affinity B cell clones. As the GC response evolved, T_FH cells extinguished IL-21 production and switched to IL-4 production, showed robust expression of the co-stimulatory molecule CD40L, and promoted the development of antibody-secreting B cells via upregulation of the transcription factor Blimp-1. Thus, T_FH cells in the B cell follicle progressively differentiate through stages of localization, cytokine production and surface ligand expression to 'fine tune' the GC reaction.