Interleukin 17A (IL-17A) is the founding member of the family of cytokines that includes IL-17B through IL-17F. It is a potent pro-inflammatory cytokine that plays a key role in defense against pathogens. IL-17A and IL-17F signal as homodimers or heterodimers through the same receptor, and activate NF-κB, MAPK, and C/EBP pathways (Gaffen). IL-17A is produced by Th17 cells, CD8+ T cells, γ/δ T cells, natural killer (NK) T cells, B cells, innate lymphoid cells, and mesenchymal stromal cells (MSCs) (Cua and Tato; Gaffen; Mojsilović et al.). IL-17A mediates protection against extracellular pathogens, and together with IL-22 stimulates production of antimicrobial peptides. It induces granulopoiesis factors and neutrophil-specific chemokines. Together with tumor necrosis factor alpha (TNF-α), IL-17A induces a sustained neutrophil recruitment during inflammation (Cua and Tato). IL-17A receptor is expressed at particularly high levels on stromal cells, including MSCs. IL-17A increases the frequency and the average size of fibroblast colony-forming units (CFU-F), as well as the proliferation of marrow-derived MSCs. It enhances osteogenic differentiation, and inhibits adipocyte differentiation and chondrogenesis (Mojsilović et al.).