A recombinant therapeutic agent which is chemically identical to or similar to the endogenous cytokine interleukin 11 (IL-11). Secreted by bone marrow stromal cells and a number of mesenchymal cells, IL-11 binds to and activates its cell-surface receptor, promoting primary and secondary immune responses, modulating antigen-specific antibody reactions, and preventing apoptotic cell death. This agent also stimulates the T-cell-dependent development of IgG-secreting B-cells in spleen cell cultures and may be an important regulator of megakaryocytopoiesis. (NCI04)