Interleukin (IL) 6 is a pleiotropic cytokine (26 kDa) that originally was named interferon beta 2 or B cell-stimulating factor or differentiating B cell factor inducing immunoglobulin production. IL-6 is produced in many diseases. After secretion, IL-6 binds to its receptor IL-6R alpha (gp 80), the IL-6R alpha complex then recruits the signal-transducing beta-subunit (gp 130), which is the functional complex for signal transduction. In addition, activation of Th2 cells or mast cells also produce IL-6, which mediates immune responses, inflammation, acute phase responses, hematopoiesis, cancer, inflammatory bowel disease, etc. IL-6 also is a crucial cytokine for mast cell maturation. Human cord blood CD34+ cells differentiate and grow into mast cells in the presence of stem cell factor (SCF) and IL-6, causing increases in cell size, frequency of chymase positive cells, and intracellular histamine levels when compared with cells treated with SCF alone. Activated mast cells increase IL-6 mRNA associated with protein kinase C (PKC) activity. IL-6 also up-regulates histamine production rather than increases its storage and is an important inducing factor for the expression of immunoglobulin E (IgE) Fc epsilon RI.