FIG. 2:

Features of innate-like effector lymphocytes mirror those of conventional T lymphocytes. Three types of effector immune responses are recognized: type 1, type 2, and type 3. They are characterized by the absence (ILCs) or presence of antigen-specific receptors on lymphoid lineage cells. Whilst conventional T cells express a diverse TCR (noted in the current IMGT nomenclature) repertoire, those of the innate-like effector lymphocytes express semi-invariant (NKT and MAIT cells) to invariant (γδT cells) TCRs. Type 1 effectors include both innate (NK) and adaptive cytotoxic (CD8⁺ T) cells and non-cytotoxic T helper (Th) 1 cells, as well as innate-like effector lymphocytes such as ILC1, NKT1, MAIT1, and γδT1 cells. They require IL-12 for induction, which is bolstered by IFN-γ. T-bet and related eomesodermin transcription factors control the differentiation of type 1 effector cells, which are essential for immunity against intracellular pathogens. Type 2 effector cells include Th2, ILC2, NKT2, and γδT cells. These cells are activated by IL-4 and require GATA3 for their effector differentiation. Type 2 effector cells secrete IL-4, IL-5, and IL-13, which are required for parasite expulsion. Their over activity results in allergic response and hypersensitivities. Type 3 effector cells include Th17, ILC3, NKT17, MAIT17, and γδT17 cells. These effector cells are induced by IL-6, TGF- β, IL-1 β, IL-23, and IL-7 (NKT17). RORγt is the lineage specific transcription factor. Type 3 effector cells secrete IL-17 and IL-22, which are important for immunity to extracellular bacteria and fungi.