Mechanism of immunity against micro-organisms 203 



prevent the micro-organisms, when united into motionless masses, 

 from being ingested by the leucocytes with greater ease. 



In the case of the typhoid bacillus, one of the most active of bacteria, 

 the same facts may be observed as in the case of the cholera vibrio. [214] 

 In animals that remain unaffected we often see the last free bacilli 

 moving about actively between the leucocytes filled with microbes. 

 In many other examples of natural immunity we constantly meet 

 with phagocytes containing but a single or a small number of micro- 

 organisms (streptococci, yeasts, etc.). 



The presence of motile micro-organisms inside phagocytes proves 

 also that it is possible for these cells to do without the help of agglu- 

 tinative substance in carrying on their protective work. The most 

 carefully studied case of the relations between natural immunity and 

 agglutination is that met with in the anthrax bacillus. We owe it to 

 Gengou 1 , who at the Liege Bacteriological Institute carried out a very 

 detailed investigation on this question. He showed that the bacillus 

 of Pasteur's first anthrax vaccine is agglutinated by the blood serum of 

 a great number of animals. But he also showed that the serums which 

 have the greatest agglutinative action on this bacillus do not come from 

 the most refractory species. Human serum agglutinates most strongly 

 the bacillus of the first vaccine (in the proportion of one part of 

 serum to 500 parts of culture) but man is far from being exempt 

 from anthrax. Pigeon's serum, on the other hand, is completely 

 without any agglutinative power, although this species resists not only 

 the first vaccine but very often even virulent anthrax. The serum 

 of the ox, a species susceptible to anthrax, is more agglutinative 

 (1 : 120) than that of the refractory dog (1 : 100). There are, how- 

 ever, exceptional cases in which the agglutinative property cor- 

 responds to the degree of susceptibility. Thus the serum of the 

 mouse has not the slightest agglutinative action on the bacillus of 

 the first vaccine. But alongside this example is that of the rat, a 

 species of moderate susceptibility to anthrax, whose serum possesses 

 the least agglutinating power of all, acting only in the proportion 

 of 1 : 10. All these facts fully justify the conclusion formulated by 

 Gengou that " we cannot establish any relation between the aggluti- 

 nating power and the refractory state of the animals to anthrax " 

 (p. 319). This conclusion may be extended to the phenomena of the 



1 Arch, internat. de Pharmacodyn., Gand et Paris, 1899, t. vi, p. 299; Ann. 

 de I'Inst. Pasteur, Paris, 1899, t. xiu, p. 642. 



