IMMUNITY 69 



test of permitting the leukocytes of the blood of the patient, 

 and as a control, those of a healthy person, to feed upon the 

 bacteria in question in test-tubes kept at body heat. If the 

 number of germs consumed by the patient's leukocytes rises 

 during the* course of the treatment, he is considered as bene- 

 fitting from the injections. His general constitutional con- 

 dition is closely watched also. It is now attempted to use for 

 "vaccination" a culture made from the patient's disease, the 

 co-called "autogenous vaccine." 



Principles of Artificial Immunity. To return for a space to 

 active immunity, it is well to consider here the basis of the 

 present-day bacterin treatment. The ordinary vaccination 

 against smallpox depends upon the fact that human smallpox 

 virus passed through a calf for a number of times loses its 

 power to produce smallpox in man. It does retain power to 

 produce a sore, and this sore contains sufficient of the poison 

 related to smallpox virus to stimulate in the vaccinated 

 person a condition resistant to the virus which would cause 

 true general smallpox. The great Pasteur found that if he 

 heated anthrax bacilli and injected them into sheep, these 

 animals became resistant to the disease anthrax. Since the 

 time of Pasteur the management of the process which has been 

 called " active immunization" has been learned. To accom- 

 plish this a virus must be treated as was the virus of smallpox, 

 that is, it must be rendered incapable of causing general 

 disease, but it must not be so altered that it has no relation 

 to its original form. The living organisms can be taken and 

 subjected to higher or lower temperatures than those pre- 

 ferred by the individual species, or they may be injected into 

 animals until they will merely live without producing disease. 

 This is called reducing virulence. They may be killed by heat 

 or obtained in mass and crushed and ground into a pulp. 

 Again, the broth or other material upon which they grow may 

 be used after removing the bacterial bodies by filtering them 

 off through porcelain filters. Having obtained the virus in a 

 reduced state either dead or as active principles, it is injected 



