Abstract
An attempt has been made to test the immunostimulation hypothesis of Prehn which proposes that, in its initial stages, the immune response to tumourβspecific antigens stimulates rather than inhibits tumor growth. Mice have been injected with cells from three different methylcholanthreneβinduced tumors and the cellular and humoral antiβtumor immune responses of their peripheral blood lymphocytes and sera have been studied by in vitro techniques. The results have been found to be completely compatible with the immunostimulation hypothesis. Five days after the inoculation of tumor cells, at a time when tumors are still not palpable, the cellular antiβtumor immune responses of the mice were found to be capable of specifically stimulating tumor growth. Palpable tumors normally appeared in these animals 7β10 days after the inoculation of tumor cells and by day 12 it was found that, in most instances, the stimulatory pattern seen on day 5 had changed to an inhibitory one. Other findings reported in this paper relate to the effect of normal and immune serum on tumor cells and on the interaction between tumor cells and normal or immune lymphocytes. Finally, on the basis of all these results, a hypothesis is presented to explain how tumors grow and why they do so in the face of an immune response directed against antigens specific to them.