Human IL-5 (also known as BCGF and BCDF) is a ~40 kDa factor produced mainly by T cells. IL-5 is present as an N-glycosylated homodimer and is a hematopoietic growth factor promoting proliferation, activation and differentiation of eosinophil granulocytes. In the human, IL-5 also supports generation of cytotoxic T cells. In the mouse, IL-5 acts on B cells to induce proliferation and secretion of IgM and IgA from endotoxin activated B cells.