Lymphokine produced by antigen‐ or mitogen‐activated T cells. Its principal role is regulation of IgEand eosinophil‐mediated immune reactions. It stimulates switching of B cells for production of immunoglobulin E (IgE), is a growth and differentiation factor for T cells (particularly helper T cells), is a growth factor for mast cells, and stimulates the expression of some adhesion molecules on endothelial cells. Formerly called B lymphocyte stimulatory factor 1.