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.