UNICELLULAR FORMS 85 



Artificial Immunity: Since the body cells respond to the pres- 

 ence of certain disease-producing bacteria by forming antibodies, 

 the question arises: Is it possible to induce these cells to produce 

 antibodies by means other than the invasion of active and virulent 

 disease-producing bacteria? Attempts to so activate body responses 

 by artificial means have met with considerable success. The reac- 

 tion may be induced in two ways: One, by the injection of a live 

 culture of the disease germs that are so weakened that they produce 

 only very mild symptoms, but their toxins are sufficient to elicit a 

 response of antibodies from the body cells. Two, by the injection of 

 a killed culture of the disease germs; their disintegrated bodies 

 liberate toxin in quantities sufficient to cause an outpouring of 

 antibodies. The first is the method employed in immunizing against 

 smallpox. Long before the bacterial cause of disease was understood 

 this method of artificial immunization had been employed by 

 Jenner in the eighteenth century. The reasons for the success of 

 Jennerian vaccination are now clear. 



Vaccines. The organism that causes smallpox is so small that 

 it has never been seen; in fact, so small that it will pass through 

 the pores of a porcelain filter. Such minute organisms are found to 

 be the cause of a number of diseases, for example, hog cholera, and 

 are called filterable viruses.^ In understanding vaccination in the 

 case of smallpox it must be remembered that bacteria, including the 

 filterable viruses, execute all the phenomena of living just as do 

 all other cells and organisms. They are subject to the same limita- 

 tions in their environment; their food supply must be adequate in 

 quantity and of certain quality, or they do not flourish; they become 

 weakened, attenuated. So in the case of the virus that causes small- 

 pox, the human body appears to be its natural environment, but it 

 will live, although weakened, in the body of the cow. A culture 

 of this attenuated strain from the cow is not potent to produce the 

 disease in man; only a small pustule develops at the point of inocu- 



^ There is considerable important evidence that the filterable viruses are quite distinct 

 from bacteria. However, this distinction primarily concerns the bacteriologist. 



