432 IMMUNITY. 



guinea-pig has acquired a high degree of immunity, and 

 Haffkine believed that this immunity was effective in the 

 case of every method of inoculation, that is, by the mouth 

 as well as by injection into the tissues. After trying his 

 method on the human subject and finding it free from 

 risk, he extended it in practice on a large scale in India in 

 1894, and these experiments are still going on. So far 

 the results are, on the whole, encouraging. In the human 

 subject two or sometimes three inoculations are made with 

 attenuated virus before the virus exalte is used. Wasser- 

 mann and Pfeiffer, and also Klein, have found, however, 

 that guinea-pigs immunised by Haffkine's method are not 

 immunised against intestinal infection when the animal is 

 treated by Koch's method (that is, by paralysing the 

 intestines with opium, vide p. 385). Notwithstanding this 

 fact Haffkine's method may still have a beneficial effect, 

 though it may not be preventive in all cases. 



2. Immunity by Dead Cultures of Bacteria. In some 

 cases a high degree of immunity against infection by a 

 given microbe may be developed by repeated and gradually 

 increasing doses of the dead cultures, the cultures being 

 killed sometimes by heat, sometimes by exposure to the 

 vapour of chloroform. Some consider that in this method 

 only the intracellular toxic substances of the organism are 

 introduced when the cultures have been taken from the 

 surfacslpf a solid medium, such as agar, but as the surface 

 is mohi, some of the extracellular products must be present 

 also, t The cultures when dead produce, of course, less 

 effect -jhan when living, and this method may be con- 

 venien> r/ used in the initial stages of active immunisation, 

 to be . erwards followed by injections of the living cultures. 

 The method has been extensively used by Pfeiffer and 

 others: in the production of a high degree of immunity 

 in guinea-pigs against the typhoid, cholera, and other 

 organisi.is. 



3. Immunity by the Separated Bacterial Products or 

 Toxines. The organisms in a virulent condition are grown 

 in a fluid medium for a certain time, and the fluid is then 



