IL-17A/F is a secreted disulfide-linked heterodimer of the cytokines IL-17A and IL-17F. IL-17A/F and the IL-17A and IL-17F homodimers are produced by IL-23 activated Th17 cells. The widely expressed receptors IL-17 RA and IL-17 RC form a heterodimer for the binding of IL-17A and IL-17F, as well as the heterodimeric IL-17A/F. IL-17A/F is a biologically active protein that induces chemokine production and airway neutrophilia with intermediate potency between IL-17A (most potent) and IL-17F (least potent). It is upregulated in immune cells during inflammatory arthritis and contributes to disease severity.