Interleukin 16 (IL-16) is a cytokine released by a variety of cells, including lymphocytes and some epithelial cells, and is a chemotactic factor for immune cells that specifically express the cell surface molecule CD4.IL-16 was initially described as a factor that attracts activated T-cells in humans and was previously referred to as a lymphocyte chemotactic factor (LCF). Since then, studies have demonstrated that IL-16 recruits and activates many other cells that express the CD4 molecule, including monocytes, eosinophils, and dendritic cells. The protein encoded by this gene is a pleiotropic cytokine that acts as a chemokine, a regulator of T cell activation, and an inhibitor of HIV replication. The signaling process of this cytokine is regulated by CD4.IL-16 expression has been associated with inflammatory processes in asthma, rheumatoid arthritis, systemic lupus erythematosus, ulcerative colitis, atopic dermatitis, and multiple sclerosis. For example, IL-16 expression is directly correlated with the number of infiltrating CD4+ T cells in asthma epithelium.