468 IMMUNITY. 



munity produced in this way is effective not only against the 

 toxin, but also against large doses of the virulent organism in a 

 living condition. This method was carried out with a great 

 degree of success in the case of diphtheria, tetanus, malignant 

 oedema, etc. It appears capable of very general apphcation, 

 though, in the case of many organisms, it is difficult to get a 

 very active toxin from the filtered cultures. It has also been 

 applied in the case of snake poisons by Calmette and Fraser, 

 and a high degree of immunity has been produced. 



Immunity may also be obtained by means of certain chemical 

 substances separated from filtered bacterial cultures, though 

 these substances are generally in a more or less impure condition. 

 Hankin was the first to obtain this result by means of an 

 albumose separated from anthrax cultures. 



Though, as already stated, none of these methods can be 

 used directly as curative agents, seeing that they imply previous 

 treatment before exposure to infection, yet they supply the 

 means of developing a very high degree of immunity, which 

 is the first stage in the production of an active curative serum. 



The following may be mentioned as some of the most 

 important examples of the practical application of the principles 

 of active immunity, i.e. of protective inoculation: (i) Inocula- 

 tion of sheep and oxen against anthrax (Pasteur) (p. 315); (2) 

 Jennerian vaccination against smallpox (p. 501); (3) Anti-cholera 

 inoculation (Haffkine) (p. 421); (4) Anti-plague inoculation 

 (Haffkine) (p. 444) ;' (5) Anti-typhoid inoculation (Wright and 

 Semple (p. 346) ; (6) Pasteur's method of inoculation against 

 hydrophobia, which involves essentially the same principles. 



Active Immunity by Feeding. — Ehrlich found that mice 

 could be gradually immunised against ricin and abrin by feeding 

 them with increasing quantities of these substances (vide p. 177). 

 In the course of some weeks' treatment in this way the resulting 

 immunity was of so high a degree that the animals could tolerate 

 400 times the dose originally fatal by subcutaneous inoculation. 

 Fraser also found in the case of snake poison that rabbits could 

 be immunised, by feeding with the poisons, against several times 

 the lethal dose of venom injected into the tissues. 



By feeding animals with dead cultures of bacteria or with 

 their separated toxins, a certain degree of immunity may in 

 certain cases be gradually developed. But this method is so 



