Interleukin 5 is a cytokine produced primarily by activated T lymphocytes. It exists as an antiparallel disulfide linked homodimeric glycoprotein with 115 amino acid residues in each chain. Known also as EDF (eosinophil differentiating factor), functions predominantly as an eosinophilopoietic factor. Analysis of its crystal structure reveals a novel two domain structure, with each domain showing significant homology to the cytokine fold in GMCSF, MCSF, IL2, IL4 and growth hormone. Human and m