Interleukin 7 (IL-7) is a hematopoietic growth factor secreted by bone marrow and thymic stromal cells, and is also produced by keratinocytes, dendritic cells, hepatocytes, neurons, and epithelial cells, but normal lymphocytes do not produce IL-7. IL-7 stimulates the differentiation of pluripotent hematopoietic stem cells to lymphoid progenitors, and similarly stimulates the proliferation of cells of all lymphoid lineages (B cells, T cells, and NK cells). IL-7 promotes the development of malignant hematologic diseases (acute lymphoblastic leukemia, T-cell lymphoma), elevated levels of IL-7 are detected in the plasma of HIV-infected patients, and IL-7 also enhances immune recovery after allogeneic stem cell transplantation.