ANTIMICROBIC SERUM. 443 



in large doses must be ultimately tolerated by the animal. 

 For this purpose a fairly virulent culture is obtained fresh 

 from a case of the particular disease, and its virulence may 

 be further increased by the method of passage. This 

 method of obtaining a high degree of immunity against the 

 microbe is specially applicable in the case of those organisms 

 which invade the tissues and multiply to a great extent 

 within the body, and of which the toxic effects, though 

 always existent, are proportionately small in relation to the 

 number of organisms present. The method has been 

 applied in the case of the typhoid and cholera organisms, 

 the bacillus of bubonic plague, the bacillus coli communis, 

 the pneumococcus, streptococcus (Marmorek), and many 

 others. In fact, it seems capable of very" general applica- 

 tion. 



The important result obtained by such experiments, is, 

 that if an animal be highly immunised by the 

 mentioned, the development of the immunity is 

 panied by the appearance in the blood of protective s 

 stances, which can be transferred to another animal. The 

 law enunciated by Behring regarding immunity against 

 toxines thus holds good in the case of the living organisms, 

 as was first shown by PfeifTer. The latter found, for 

 example, that in the case of the cholera organism, so high 

 a degree of immunity could be produced in the guinea-pig, 

 that .002 c.c. of its serum would protect another guinea- 

 pig against ten times the lethal dose of the organisms, when 

 injected along with them. Here again is presented the 

 remarkable potency of the antagonising substances in the 

 serum, which in this case lead to the destruction of the 

 corresponding microbe. 



The anti-streptococcic serum of Marmorek may be briefly 

 described, as it has come into extensive practical use. 

 This observer found that he could intensify the virulence 

 of a streptococcus by growing it alternately in the peritoneal 

 cavity of a guinea-pig, and in a mixture of human blood 

 serum and bouillon (vide p. 155). The virulence became 

 so enormously increased by this method that when only 



