Interleukin-10 (IL-10), initially known as Cytokine Synthesis Inhibitory Factor (CSIF), belongs to the IL-10 family and shares more than 80% sequence homology with the Epstein-Barr Virus protein BCRFI. It is produced by many immune cells, such as T-cells, macrophages, mast cells and dendritic cells. It is usually secreted as a homodimer and, upon binding to its receptor, inhibits the synthesis of a number of cytokines, including IFN-gamma, IL-2, IL-3, TNF and GM-CSF, by activated macrophages and Th2 cells. It also displays the ability to suppress Antigen-Presenting Cell (APC) function. The net effect of Interleukin-10 appears to be inhibitory; however, stimulatory effects, such as stimulation of B cell maturation and antibody production, are also reported.