RESUMO
T lymphocytes consist of several subtypes with distinct functions that help to coordinate an immune response. They are generated within the thymus through a sequential developmental pathway that produces subsets with diverse antigen specificities and functions. Naïve T cells populate peripheral lymphoid organs and are activated upon foreign antigen encounter. While most T cells die soon after activation, a memory population survives and is able to quickly respond to secondary challenges, thus providing long-term immunity to the host. Although cell identity is largely stable and is instructed by cell-specific transcriptional programs, cells may change their transcriptional profiles to be able to adapt to new functionalities. Central to these dynamic processes are transcription factors, which control cell fate decisions, through direct regulation of gene expression. In this book chapter, we review the functions of the transcription factor B-cell lymphoma 6 (BCL6), which directs the fate of several lymphocyte subsets, including helper, cytotoxic, and innate-like T cells, but can also be involved in lymphomagenesis in humans.