Publisher Summary This chapter focuses on properties and functions of interleukin-10. IL10 inhibits several macrophage functions, including some microbicidal properties and presentation of antigen to TH 1 cells. In contrast, IL10 has generally enhancing or stimulatory functions on B cells and mast cells. Because IL10 is produced by macrophages and other cell types in addition to the T cells from which it was originally identified, it is clear that IL10, in common with several other cytokines, has a much more complex role in the immune system than could be inferred from the original activity. IL10 inhibits the synthesis of several cytokines that are normally secreted by human and mouse monocytes/macrophages in response to activation by lipopolysaccharide. Several functions of IL10 are centered on inhibition of macrophage activation and function. TH1-like responses are in general inhibited by IL10, which appears to be a consistent part of strong TH2 responses against a variety of pathogens and in several other disease states. The early results regarding IL10 production and IL10 interventions in vivo suggest that IL10 can be deleterious to situations in which a delayed type hypersensitivity response is required, for example many intracellular pathogen infections.