Interleukin-7 (IL-7) is a hematopoietic cytokine that is an important regulator of B and T cell development. IL-7 is secreted by bone marrow and thymic stromal cells, dendritic cells, intestinal epithelial cells, hepatocytes, and keratinocytes. IL-7 signals through the interleukin-7 receptor (IL-7R) to promote the differentiation of hematopoietic stem cells into T cells, B cells, and Natural Killer cells. IL-7 is also a regulator of intestinal mucosal lymphocyte proliferation. Human and mouse IL-7 show species cross-reactivity.