IL-17F, which is closely related to IL-17A, is known to be expressed by T helper 17 (TH 17) cells, CD8+ cells, γδ T cells, NK cells, NKT cells, and LTi cells. Like IL-17A, IL-17F has been shown to form biologically active homodimers (IL-17FF) and heterodimers (IL-17AF). IL-17AA, IL-17FF, and IL-17AF are currently believed to signal through the same receptor complex (IL-17R) composed of the subunits IL-17RA and IL-17RC (IL-17RL), though they appear to have different biological functions. It has been suggested that differential expression patterns of the IL-17R subunits could provide a tissue specific signaling mechanism for IL-17A and IL-17F. Accordingly, the signal elicited upon binding by either IL-17A or IL-17F is remarkably different. The functions of IL-17F are believed to be in neutrophil recruitment and immunity to extracellular pathogens. IL-17F treatment of airway epithelium, vein endothelial cells, and fibroblasts has been reported to induce expression of IL-6, IL-8, GROα, ENA-78, TGFβ, MCP-1, G-CSF, GM-CSF, and ICAM-1.