IL-16 is an immunomodulatory cytokine secreted by activated CD8+ T cells, as well as other immune and non-immune cell types. It serves as a chemoattractant for CD4+ T cells, macrophages, and eosinophils. IL-16 induces expression of CD25 (IL-2R) and plays a role in the accumulation and activation of CD4+ cells recruited to sites of inflammation. IL-16 (121 a.a.) is a proteolytic product of pro-IL-16 cleaved by Caspase-3 after T-cell activation, producing a secreted biologically active form. It requires surface CD4 to mediate signaling processes.