Page:The Scientific Monthly vol. 3.djvu/226

 220 THE SCIENTIFIC MONTHLY

been our main consideration^ and the experimental cancer. In the latter we have to deal in almost all cases with a transfer of can- cerous cells from one animal to another^ while in the spontaneous can- cer some of the cells of the animal itself become cancerous. In experi- mental cancer^ then^ usually cells of another animal grow within the inoculated individual. This distinction is of great importance when we come to consider the defensive mechanisms that arise within the body of the animal against the inroads of the cancer cells. We may briefly point out that spontaneous defensive mechanisms arise only in the case of experimental tumors (experimental tumors are foreign ccUb introduced into another organism and growing there), the body does not react defensively against spontaneous tumors. It is important to note that the defensive mechanism provided by the body against the attack of cancer cells can be artificially increased. This ability to bring into action defensive mechanisms we call immunity. The immunity produced in the case of experimental tumors, is, under ordinary condi- tions, not an inmiunity directed against the causative agent of the tumor associated with the cells and being either, as we have seen, a cer- tain chemical body or a microorganism, but against the transplanted cells. Usually no immunity can be produced in the organism in which the cells have originated. But immunity can be produced against cells that are foreign to the organism — transplanted into it. The phe- nomena with which we deal in the process of tumor immunity are therefore not peculiar to tumors, but are the same in every case of transplantation of tissues. No defensive mechanism is developed in the individual against the cells of his own body, only against foreign cells. Thus if we excise a bit of skin or of the thyroid gland or of the uterus and place it in a pocket under the skin, the tissue remains pre- served almost intact for a long period, probably during the entire life- time of the individual. But if, on the other hand, we transplant a piece of skin from another individual of the same species, we find that the tissue is, after a longer or shorter period, destroyed within the new host. If we make careful nucroscopic examinations of tissue trans- planted into a new host, we find that certain cells which circulate in the blood and lymph emigrate from the vascular channels of the organ- ism in order to invade and gradually destroy the transplanted tissue. Other cells of the host also exert a deleterious effect on the foreign tissue and contribute to its final destruction.

If, instead of transplanting tissue from one individual to another of the same species, we inoculate an individual of a different species the tissue usually perishes directly through the influence of the body fluids which are poisonous for the transplanted tissue.

How can we explain these facts ? We must assume that every indi- vidual of a certain species differs in a definite chemical way from every

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