TEE SCIENTIFIC INVE8TI0ATI0N OF CANCER 223
against which we have to protect the animal are the cancer cells taken from another individual which grow in the new host If, instead of inoculating bacteria we inoculate normal tissue cells from another in- dividual of the same species or tumor cells which happen not to grow in the inoculated animal, we find that after a certain time a kind of immunity is produced in that animal. If later we inoculate that ani- mal with fully virulent tumor cells, this tumor very frequently does not grow. As we stated before, the majority of spontaneous tumors can not very well be transplanted. It is therefore comparatively easy to find a tumor which does not grow after inoculation, but which, neverthe- less, forms a protective substance fatal to the growth of a virulent tumor. Now, while it is frequently possible to immunize against bacteria with dead bacteria or with certain extracts of bacteria, this is not possible in cancer immunization. Here we have to inoculate with living ceUs which only gradually die if transferred into the new host. Apparently a certain product of metabolism of the introduced cells, perhaps some product of oxidation, is necessary for this purpose, and such an oxida- tive process seems to take place only in the living cell.
How can we then interpret the mechanism which leads to the set- ting at work of these protective agencies? The most probable explana- tion is as follows: If the cell from the same species, but containing a differential chemical substance, is introduced into another individual, it attracts either directly certain wandering cells of the blood, which invade the tumor and also cause the connective tissue cells to form denser fibrous- bands around the tumor, thus cutting off the nourish- ment from the transplanted cells, or such an effect is produced in an indirect manner. The differential chemical substance in the cells calls forth the new production of a chemical immunizing substance which circulates in the blood, or calls into action a substance which was already preformed in an animal and which it was not necessary to produce artificially. But even in the latter case an additional new formation of such an immunizing substance is added to the first sub- stance which was preformed; these substances act upon the introduced cells, change their metabolism in a more or less injurious manner and call forth the reaction on the part of the wandering cells of the blood and of the connective tissue cells. We find, therefore, that in the case of tumors the reaction is exactly the same as with the transplanted normal tissue.
As stated before, after the so-called homoiotransplantation of tissue — a transplantation into another individual of the same species — cer- tain wandering cells of the blood as well as the connective tissue are the principal agents in destroying the invading tissue. But just as in the case of auto-transplantation of normal tissue — ^a transplantation into the same individual in which it originated — ^no antagonistic re- action is called forth on the part of the wandering cells and connective
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