IL-16 is a pleiotropic cytokine that functions as a chemoattractant,a modulator of T cell activation,and an inhibitor of HIV replication. It is mainly produced by T lymphocytes but also by other immune cells,neuronal cells,fibroblasts and epithelial cells. The signaling process of this cytokine is mediated by CD4. IL-16 induces chemotaxis of CD4+ cells such as lymphocytes,eosinophils,and dendritic cells by ligating CD4 directly at a site distinct from other ligands. Among its multiple functions,IL-16 is a T cell chemoattractant involved in T helper cell inflammatory responses and the regulation of both T cell growth,and responsiveness to regulatory cytokines. The cytokine function is exclusively attributed to the secreted C-terminal peptide,while the N-terminal product may play a role in cell cycle control.