The interleukin 17 (IL-17) family of cytokines contains 6 structurally related cytokines, IL-17A, IL-17B, IL-17C, IL-17D, IL-17E and IL-17F. IL-17 family plays crucial roles in host defense against microbial organisms and in the development of inflammatory diseases. IL-17A is a pro-inflammatory cytokine that also has the capacity to promote angiogenesis and osteoclastogenesis. IL-17F shares the highest homology with IL-17A and signals via a receptor composed by the IL-17RA and IL-17RC subunits. IL-17A and IL-17F can form IL-17A/A or IL-17F/F homodimers, IL-17A/F heterodimers are also formed. IL-17A and IL-17F, produced by the Th17 CD4(+) T cell lineage, have been linked to a variety of inflammatory and autoimmune conditions. IL-17F levels are elevated in sera and lesional psoriatic skin compared to non-lesional tissue. IL-17F also has been implicated in the development of neutrophilic airway inflammation.