T-bet is expressed in multiple cells of the innate and adaptive immune system. Its expression is required for the survival, development and proper functions of immune cells. In the innate immune system, T-bet is expressed in dendritic cells (DCs), natural killer (NK) cells, natural killer T (NKT) cells and innate lymphoid cells (ILCs). In the adaptive immune system, T-bet is expressed in CD4+ and CD8+ T effector cells, B cells, γδ T cells and a subset of regulatory T (TReg) cells. IFNγ, interferon-γ; IL-7Rα, interleukin-7 receptor-α; NKp46, NK cell p46-related protein; RORγt, retinoic acid receptor-related orphan receptor-γt; TFH, T follicular helper; TH, T helper; TNF, tumour necrosis factor.