IL-17 family members are glycoproteins secreted as dimers that induce local cytokine production and recruit granulocytes to sites of inflammation. IL-17 is induced by IL-15 and IL-23, mainly in activated CD4 T cells distinct from Th1 or Th2 cells. Of the six IL-17 family members, IL-17A and IL-17F share the strongest homology (50% amino acid identity) and the two genes are located in the same chromosomal region. IL-17A and IL-17F have been observed in tissue samples from various autoimmune diseases, such as rheumatoid arthritis, multiple sclerosis, psoriasis, inflammatory bowel disease, and asthma. IL-17A expression is associated with many inflammatory diseases in humans, such as rheumatoid arthritis, multiple sclerosis, asthma, systemic lupus erythematosus, and allograft rejection. Expression of IL-17F is also linked with human inflammatory diseases, including asthma.