In humans, IL10 is produced by activated CD8(+) peripheral blood T-cells, by T-helper CD4(+) T cell clones after both antigen-specific and polyclonal activation, by B cell lymphomas, and by monocytes following cell activation by bacterial lipopolysaccharides and mast cells. Human IL10 is a homodimeric protein that shares 73% amino acid homology with mouse IL10. IL10 inhibits the synthesis of a number of cytokines such as IFN-gamma, IL2 and TNF-beta in Th1 T helper subpopulations of T cells but not of Th2 helper cells. This activity is antagonized by IL4. The inhibitory effect on IFN-gamma production is indirect and appears to be the result of a suppression of IL12 synthesis by accessory cells. In the human system, IL10 is produced by, and down-regulates the function of Th1 and Th2 cells.