IMMUNITY. 419 



to cholera, and the facts mentioned above have, in our 

 opinion, great weight in establishing the relation of the 

 organism to the disease. 



Immunity. As this subject is discussed later, only a 

 few facts will be here stated, chiefly for the purpose of 

 making clear what follows with regard to the means of 

 distinguishing the cholera spirillum from other organisms. 

 The guinea-pig or any other animal may be easily immunised 

 against the cholera organism by repeated injections (con- 

 veniently made into the peritoneum) of non-fatal doses of 

 the spirilla. It is better to commence the process with 

 non-fatal doses of cultures killed by the vapour of chloroform 

 or by heat, the doses being gradually increased, and after- 

 wards to proceed with increasing doses of the living organism. 

 In this way a high degree of immunity against the organism 

 is developed, and further, the blood serum of an animal thus 

 immunised (anti- cholera serum) has markedly protective 

 power when injected, even in a small quantity, into a 

 guinea-pig along with five or ten times the fatal dose of the 

 living organism. The blood serum of an animal immunised 

 against the cholera organism has, however, no special pro- 

 tecting power against another species of organism. This 

 constitutes the principle of Pfeiffer's method of diagnosis to 

 be described. 



A curious fact, however, is, that immunity produced by the above 

 method is only exerted against the living organisms, and does not pro- 

 tect against the toxines, that is, it is due to certain substances which 

 act as germicides (indirectly), but which are not antitoxic. Further, it 

 does not protect the guinea-pig from the intestinal infection by Koch's 

 method (Pfeiffer and Wassermann, Sobernheim, Klein), nor does the 

 anti-cholera serum protect young rabbits against the choleraic affection 

 produced by ingestion of the cholera vibrios (Metchnikoft). The infer- 

 ence from these latter results appears to be, that when the vibrios are 

 introduced into the tissues, they are killed by certain substances in the 

 serum, but in the intestine they are in a sense outside of the tissues, 

 and can there multiply and produce toxines. Metchnikoff has pre- 

 pared a true antitoxic cholera serum by injections of repeated and 

 gradually increased doses of the toxine, and has found that this anti- 

 toxic serum has a distinct effect against the choleraic affection of 

 rabbits. 



