Interleukin-7 (IL-7) is a non-redundant, pleiotropic cytokine fundamental to the development, homeostasis, and function of the immune system. It is primarily produced by stromal cells in the bone marrow and thymus, as well as by epithelial cells in peripheral lymphoid organs. IL-7 signals through a heterodimeric receptor consisting of the private IL-7 receptor alpha chain (IL-7Rα/CD127) and the common gamma chain (γc), activating key pathways such as JAK/STAT, PI3K/Akt, and MAPK to promote lymphocyte survival, proliferation, and metabolic fitness. Its critical role is underscored by the severe lymphopenia and immunodeficiency observed in both IL-7- and IL-7R-deficient humans and mice.