Primitive cytokines: harbingers of vertebrate defense.

The evolution of the immune system has awarded cytokines a key role as coordinators of the immune response. The exquisite action of cytokines in fine tuning and controlling the response raises the question of whether or not these molecules have been highly conserved through evolution. Gregory Beck and Gail Habicht have isolated and characterized two major cytokines, interleukin 1 and tumor necrosis factor, from invertebrates. In this article, they speculate on the possible function of these molecules and on the existence of other cytokines in invertebrates.