Human interleukin 27 (IL-27) p28 is a 28 kDa glycoprotein of 243 amino acids, a non-covalently linked heterodimeric cytokine that is structurally related to IL-12. The heterodimeric cytokine consists of an Epstein-Barr virus-induced molecule 3 (EBI3) subunit linked with the IL-27 p28 subunit. IL-27 p28 is the type I cytokine family and is homologous to IL-12 p35, IL-23 p19, and IL-6. It is homologous to the p40 subunit of IL-12 and IL-23 and to the extracellular domain of IL-6 Rα. EBI3 also heterodimerizes with IL-12 p35 and can exist as a homodimer. IL-27 is produced by activated antigen presenting cells including monocytes, endothelial cells, and dendritic cells. Its expression is induced in response to various inflammatory stimuli. IL-27 triggers signals through a heterodimeric receptor complex consisting of WSX-1 (TCCR) and gp130 both of which belong to the cytokine receptor superfamily. WSX-1 is specific for IL-27 and is expressed on resting/naive CD4+ T cells, CD8+ T cells, NK cells, dendritic cells, monocytes, mast cells, and B cells. IL-27 has both pro- and anti-inflammatory properties. In response to infection, IL-27 induces monocytes and mast cells to secrete pro-inflammatory cytokines. It induces naive CD4+ T cells to proliferate and develop Th1 cell responses. IL-27 also promotes effector functions of NK cells and CD8+ T cells. As an anti-inflammatory immunomodulator, IL-27 has been found to have the ability to inhibit Th1 or Th2 responses and restrict the strength and duration of adaptive immune responses.