IMMUNIZATION OF ANIMALS. 531 



conclusively that the streptococci taken up by poly- 

 morphonuclear leucocytes may be killed by the lat- 

 ter. Hence the evidence in favor of a protective 

 role by the leucocytes is more than presumptive. 

 Ruediger suggests the importance of the leuco- 

 cytic toxin of the streptococcus for the develop- 

 ment of infection. It may either kill the leuco- 

 cytes or cause negative chemotaxis, and under these 

 conditions proliferation of the cocci may proceed. 



Weaver, Tunnicliff and Boughton have shown 

 that the defense of the body against streptococci 

 depends on the power of phagocytosis on the part 

 of the leucocytes as well as the opsonin, hence the 

 estimation of the resisting power of the body must 

 be measured by the bactericidal power of whole 

 blood. 



The streptococcus usually is classed with those Acquired 

 organisms, infection with which does not cause the 

 development of lasting immunity. A certain 

 amount of immunity probably is established, how- 

 ever. This is suggested by the results of Fehlei- 

 sen, who could not always cause second attacks 

 of erysipelas by the inoculation of pure cultures 

 into the susceptible. It is also suggested by the 

 ease with which relatively high resistance can be 

 produced in animals by brief immunization. A 

 streptococcus infection of the horse which occurs 

 naturally ("Druse") is said to produce immunity 

 which lasts for a year or two. 



One may immunize animals either with toxic 

 filtrates or with killed and living cultures. The 

 filtrates are much less effective in producing im- 

 munity than the bacterial cells, and in the hands 

 of many no immunity whatever could be estab- 

 lished. 



