Schematic model of the mechanism by which serum IL-6 is increased when IL-6 receptor is blocked by tocilizumab. The bathtub model explains the elimination of IL-6 from serum before and after administration of tocilizumab. The rate of water flowing from the faucet into the tub (the IL-6 production rate) remains constant. Before tocilizumab administration, the rate of water flowing out of the bathtub (the elimination of receptor-bound IL-6 from serum and IL-6 catabolism) is also constant, so the water level (serum IL-6 level) remains constant. In the second diagram, the flow of IL-6 from the bathtub is greatly restricted by a “plug” (IL-6R–mediated elimination is inhibited by tocilizumab). The water level increases and then remains constant at a higher level (serum IL-6 increases to a new steady-state level when the IL-6 production rate matches the IL-6 degradation rate).