The finding that innate lymphoid cells can control the activity of CD4+ T cells reveals another potential form of immune-system regulation, and may help to explain how the body distinguishes resident from pathogenic bacteria. See Letter p.113 The recently characterized innate lymphoid cells (ILCs) can be classified functionally into three groups. Group 1 ILCs produce interferon-γ, group 2 cells express interleukin (IL)-5, IL-13 and amphiregulin, and group 3 ILCs produce IL-17A and IL-22. The function of ILCs in the presence of adaptive immunity and their potential to influence adaptive immune cell responses are largely unknown. A study in mice now shows that group 3 ILCs process and present antigen and control CD4+ T-cell responses to intestinal commensal bacteria through an MHC-class-II-dependent mechanism. This finding may be relevant to the pathogenesis of chronic human diseases associated with inflammatory host immune responses to commensal bacteria.