Interleukin 6: A Multifunctional Cytokine Regulating Immune Reactions and the Acute Phase Protein Response

Cytokines are protein mediators of cell-to-cell communication important in a variety of physiologic and pathophysiologic processes. Only a decade ago information about cytokines was limited to the description of a multitude of ill-defined “factors” present in crude super- natants of cultures of mononuclear cells. Modern techniques of protein chemistry and molecular biology have led to the isolation and precise functional definition of many cytokines, including the interferons, colony-stimulating factors, and a heterogeneous group of agents termed interleukins. The name “interleukins” was originally conceived to designate proteins produced by lymphocytes or monocytes, affecting the growth and/or differentiation of lymphoid, monocytic or myeloid cells. With time it has become apparent that many interleukins are not produced exclusively by white blood cells, nor are their actions restricted to leukocytes. The terms “interleukin” or “lymphokine” are now being used for cytokines that can be produced by a variety of cell types and that can affect the functions of many nonhematopoietic cells.