Effector cytokine of innate and adaptive immune system involved in antimicrobial host defense and maintenance of tissue integrity (PubMed:23255360, PubMed:18025225, PubMed:19144317). IL17A-IL17F signals via IL17RA-IL17RC heterodimeric receptor complex, triggering homotypic interaction of IL17RA and IL17RC chains with TRAF3IP2 adapter through SEFIR domains. This leads to downstream TRAF6-mediated activation of NF-kappa-B and MAPkinase pathways ultimately resulting in transcriptional activation of cytokines, chemokines, antimicrobial peptides and matrix metalloproteinases, with potential strong immune inflammation (PubMed:17911633, PubMed:15477493, PubMed:18025225). IL17A-IL17F is primarily involved in host defense against extracellular bacteria and fungi by inducing neutrophilic inflammation (PubMed:18025225, PubMed:23255360). As signature effector cytokine of T-helper 17 cells (Th17), primarily induces neutrophil activation and recruitment at infection and inflammatory sites (PubMed:18025225). Stimulates the production of antimicrobial beta-defensins DEFB1, DEFB103A, and DEFB104A by mucosal epithelial cells, limiting the entry of microbes through the epithelial barriers (PubMed:19144317). IL17F homodimer can signal via IL17RC homodimeric receptor complex, triggering downstream activation of TRAF6 and NF-kappa-B signaling pathway (PubMed:28813677). Via IL17RC induces transcriptional activation of IL33, a potent cytokine that stimulates group 2 innate lymphoid cells and adaptive T-helper 2 cells involved in pulmonary allergic response to fungi (PubMed:28813677). Likely via IL17RC, promotes sympathetic innervation of peripheral organs by coordinating the communication between gamma-delta T cells and parenchymal cells. Stimulates sympathetic innervation of thermogenic adipose tissue by driving TGFB1 expression (PubMed:32076265). Regulates the composition of intestinal microbiota and immune tolerance by inducing antimicrobial proteins that specifically control the growth of commensal Firmicutes and Bacteroidetes (PubMed:29915298). Homodimer; disulfide-linked (By similarity). Heterodimer with IL17A (IL17A-IL17F) (PubMed:18025225). Forms complexes with IL17RA and IL17RC receptors with 2:1 binding stoichiometry: two receptor chains for one interleukin molecule (By similarity). IL17F homodimer forms predominantly complexes with IL17RC homodimer, whereas IL17A-IL17F favors complexes with IL17RA-IL17RC (By similarity). IL17RA and IL17RC chains cannot distinguish between IL17A and IL17F molecules, potentially enabling the formation of topologically distinct complexes (By similarity). Expressed by T-helper 17 cells (Th17) (at protein level). The expression pattern reflects the differentiation state. In fully differentiated Th17 cells, IL17A-IL17F heterodimers are produced at higher levels than IL17A-IL17A and IL17F-IL17F dimers (PubMed:18025225). Dominantly secreted in intestine (PubMed:29915298). Expressed by resident cells of the lamina propria, both epithelial cells and immune cell subsets including natural killer cells, dendritic cells, macrophages and various T and B cell subsets (PubMed:29915298, PubMed:16990136). Expressed by epithelial cells and innate immune cells in the colon (PubMed:19144317). Expressed in group 3 innate lymphoid cells (PubMed:23255360, PubMed:29915298). Induced upon antigen receptor binding in the presence of IL6 and TGB1 (PubMed:16990136, PubMed:18025225). Up-regulated by IL23A-IL12B, IL1B and TNF and inhibited by IFNG and IL4 in CD4-positive T cells (PubMed:18025225). Induced upon fungal infection in innate lymphoid cells (PubMed:23255360). Induced in lung epithelial cells upon bacterial and fungal infection (PubMed:28813677). Induced in brown adipose tissue upon cold exposure (PubMed:32076265). Mutant mice are born healthy at the expected Mendelian ratio, are fertile, and have no apparent phenotypic abnormalities (PubMed:19144317). They show increased susceptibility to S. aureus upper respiratory infection (PubMed:28813677). Mutant mice are protected from chemically induced colitis, a model of human inflammatory bowel disease (PubMed:29915298). Belongs to the IL-17 family.