274 THE PRINCIPLES OF IMMUNOLOGY 



accepted that the inoculation of microbes into the already infected 

 system is as illogical as to instil further poison into an already poisoned 

 body. However, a wide field for prophylactic vaccination is still open. 

 Soon after Wright's work bacterial vaccines were applied in every 

 conceivable way and unfortunately much harm has been done to the 

 rational use of vaccines by reckless commercialism. 



Wright and his collaborators have studied carefully the opsonic 

 index of patients the victims of infectious disease as well as that of 

 normal individuals. They found that phagocytosis is often depressed 

 in those who are unsuccessfully combating certain disease and that the 

 phagocytic power can be increased by specific bacterial vaccination. 

 They pointed out further that following the first dose of vaccine the 

 opsonic index is considerably depressed and spoke of this phenomenon 

 as the negative phase. This phase may last for several days and 

 numerous writers have thought that such a depression of phagocytic 

 resistance might indicate such a decrease of general immunity as to 

 render vaccination during an epidemic highly undesirable. The nega- 

 tive phase has been carefully investigated and many now believe that 

 it does not exist. The factor of error in the determination of the 

 opsonic index is considerable, owing to the variability of conditions 

 operating in vitro. Therefore, it is possible that the decrease of index 

 pointed to by Wright may fall within the limit of experimental error. 

 The recent observations of Balteano and Lupu indicate that no such 

 negative phase is demonstrable in cholera, and the careful investiga- 

 tions of Cantacuzene indicate that the negative phase does not occur 

 in other diseases. 



Types of Vaccines. Living Vaccines. From animal experiments 

 it is generally admitted that the greatest and most lasting immunity is 

 produced by the injection of living bacteria. The killing of bacteria 

 apparently destroys certain thermolabile substances which possess anti- 

 genie properties. In human practice the use of living bacteria is not 

 without danger. One may at first inoculate with a single living organ- 

 ism and cautiously increase the number, but the virulence of the organ- 

 ism is not easily controlled and may be so great as to make such 

 inoculations dangerous. In addition there is a risk of establishing a 

 " carrier state " since the gradual increase of the number of organisms 

 may establish a mutual immunity on the part of both the parasite and 

 the host. If the virus of the disease can be so attenuated that danger 

 of producing an outspoken attack of the disease is eliminated, vac- 

 cination can be performed with great success. The outstanding ex- 

 amples of this method in human medicine are vaccination against 

 smallpox and against rabies. In smallpox the virus is attenuated 

 by animal passage through the calf and in rabies the virus is attenuated 

 by desiccation. 



Sensitised Vaccines. These are bacterial vaccines composed of 

 "bacteria which have been exposed to their specific immune serum. As 

 early as 1891 Babes mixed the blood of a highly refractory dog with 

 an emulsion of street virus in order to produce in other animals a more 



