Table 1.

Key molecular characteristics contributing to differential roles of type I, II, and III IFNs^a

IFN type	Type I	Type II	Type III
Receptor	IFNAR1, IFNAR2 [9]	IFNGR1, IFNGR2 [125]	IL-28Rα/IFNLR1, IL-10R2 [16]
Receptor expression	Ubiquitous [126]	Diverse, especially hematopoietic cells [127]	Epithelial cells, peripheral blood lymphocytes, neutrophils, plasmacytoid DCs [16,17,102]
Members in mice and humans	Mice:  14 IFN-α  IFN-β  IFN-ε  IFN-κ  IFN-ζ Humans:  13 IFN-α  IFN-β  IFN-ε  IFN-ω  IFN-κ [5]	Mice:  IFN-γ Humans:  IFN-γ	Mice:  IFN-λ2, IFN-λ3 [128,129] Humans:  IFN-λ1, IFN-λ2, IFN-λ3  IFN-λ4 (variably pseudogenic) [130,131]
Member expression	IFN-α/IFN-β:  Plasmacytoid DCs  Macrophages  Fibroblasts  Epithelial cells [7] Others:  Female reproductive tract (IFN-ε) [77]  Keratinocytes (IFN-κ) [132]	NK cells T cells Type I innate lymphoid cells [11]	Epithelial cells of intestine and lung [133,134] Plasmacytoid DCs [135]
Downstream signaling complex	ISGF3 complex (STAT1/STAT2/IRF9) [125]	STAT1 homodimers [125]	ISGF3 complex (STAT1/STAT2/IRF9) [16]
Key activators	IRF3, IRF5, IRF7, IRF8, various STAT TFs [2]	AP-1, CREB/ATF, NFAT, T-bet, Eomes, various STAT TFs [2]	IRF1, IRF3, IRF7, NF-κB. various STAT TFs [2]

^aAbbreviations: DCs, dendritic cells; TFs, transcription factors.