Two competing influences that may explain concomitant tumor resistance.

A second tumor inoculum is often inhibited in its growth by the presence in the recipient animal of an earlier implanted, growing tumor. The tumor resulting from the first inoculum may, paradoxically, continue to grow despite the simultaneous inhibition of the growth of the second inoculum, a phenomenon usually called "concomitant immunity." Evidence now suggests that the phenomenon can be observed in the absence of any recognizable type of immune reaction and might often be named more appropriately "concomitant tumor resistance." Consideration of a variety of probably related observations suggests that concomitant tumor resistance can best be explained by the competitive interaction of two opposing influences: a local diffusible, tumor-facilitating environment, produced by both tumor and normal tissues, that is counteracted by circulating inhibitors that are also produced by both tumor and by normal tissues. In an implanted small tumor, because of geometric considerations and diffusion, the action of the local facilitating environment is weak; in a larger tumor the local facilitating environment has a relatively greater influence and thus the larger tumor can continue to grow despite levels of circulating tumor inhibitors capable of inhibiting the smaller growth.