IL-23 is a member of the IL-6 family of cytokines, and it is constituted by two subunits, p19 and p40. The p19-p40 heterodimer is stabilized by a disulfide bond. The subunit p40 is shared by IL-23 and IL-12 cytokines. p19 mRNA is expressed in endothelial cells and polarized T cells; nevertheless, p40 is not expressed by these cells. Therefore, the availability of functional IL-23 is limited by the expression of p40 and not p19. IL-23 exerts its biological activities through the interaction with a heterodimeric receptor complex composed of IL-12Rb1 and IL-23R. IL-23 activates Janus kinase (JAK)/signal transducer and activator of transcription signaling molecules (STAT). JAK2 is constitutively associated with the IL-23R chain, and binding of IL-23 to its receptor leads to phosphorylation of STAT1, STAT3, STAT4, and STAT5.