Target cells of IL-6 and the IL-6/soluble IL-6R complex. (A) Cells that express gp130 (gray) and membrane bound IL-6R (black) are responsive to IL-6 and are sensitized by the presence of the soluble IL-6R. (B) Cells that only express gp130 but no membrane bound IL-6R are unresponsive to IL-6 but can be stimulated by the complex of IL-6 and soluble IL-6R.