gp130 stimulation via soluble or membrane-bound IL-6R. (A) A donor cell (bottom) releases the soluble IL-6R that, in the presence of IL-6, stimulates the target cell (top) to dimerize gp130 (gray) and initiate signal transduction. (B) The contact between donor cell (bottom) and target cell (top) is mediated by the membrane-bound IL-6R of the donor cell, IL-6, and the two gp130 molecules of the target cell leading to gp130 dimerization and signaling. For reasons of simplicity, on donor cells, gp130 has been omitted.