570 IMMUNITY 



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 organ- 

 isms, the bacillus of bubonic plague, the bacillus coli communis, 

 the pneumococcus, streptococcus (Marmorek), and many others. 

 In fact, it seems capable of very general application. 



The important result obtained by such experiments is, that if 

 an animal be highly immunised by the method mentioned, the 

 development of the immunity is accompanied by the appearance 

 in the blood of protective substances, which can be transferred to 

 another animal. The law enunciated by Behring regarding 

 immunity against toxins thus holds good in the case of the 

 living organisms, as was first shown by Pfeiffer. The latter 

 found, for example, that in the case of the cholera organisms, 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-streptococtic 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. 41). The virulence became so enormously 

 increased by this method, that when only one or two organisms were 

 introduced into the tissues of a rabbit a rapidly fatal septicaemia was 

 produced. Streptococci of this high degree of virulence were used first 

 by subcutaneous, afterwards by intravenous injection, to develop a high 

 degree of resistance in the horse. Injections were continued over a con- 

 siderable period of time, and the protective power of the serum was 

 tested by mixing it with a certain dose of the virulent organisms, and 

 then injecting into a rabbit. The serum of a horse highly immunised in 

 this way constitutes the anti-streptococcic serum which has been exten- 

 sively used in many cases of streptococcic invasion in the human subject. 

 Marmorek, however, found that this serum had little antitoxic power, 

 that is, could only protect from a comparatively small dose of toxin 

 obtained by filtration of cultures. 



Anti-typhoid, anti-cholera, 1 anti-pneumococcic, anti-meningo- 

 coccic, anti-plague, and other sera are all prepared in an 

 analogous manner. 



1 A true antitoxic cholera serum was prepared by Metchnikoff, E, Roux, 

 and Taurelli-Salimbeni. 



