Originally known as lymphocyte chemoattractant factor (LCF), Interleukin-16 (IL-16) is a product of human peripheral mononuclear cells when antigen-stimulated or mitogen-stimulated. It is also produced by dendritic cells, epithelial cells, eosinophils, fibroblasts and mast cells. The pro-form of IL-16 is a 631 amino acid protein that is cleaved by caspase 3 in activated CD4+ T cells where it is then secreted and is accessible for binding CD4 receptors. IL-16 regulates and induces migratory response in CD4+ lymphocytes, eosinophils, and monocytes. It is a soluble ligand for CD4 and functions to prime responsiveness to IL-2 and IL-15 in CD4+ T-cells. Although Isoform 1’s function is thought to be as an anchor to membrane ion channels, Isoform 3 is more widely researched and characterized as component of T-cell cycle progression.