PASSIVE IMMUNITY 557 



obtained from the use of sensitised vaccines (vide supra). These 

 are prepared by subjecting living cultures (preferably of auto- 

 genous strains) to the action of say 5 c.c. of the appropriate 

 anti-serum for three hours at 37 C. The sensitised bacteria 

 are deposited by centrifuging, emulsified in saline containing 0*5 

 per cent, phenol, and again kept at 37 C. for three hours, so 

 that the phenol may kill them. The vaccine is then ready for 

 use. In infections with streptococci or the b. coli from ten to 

 forty millions may be given, the dose being repeated in twenty- 

 four hours. It is difficult to see the possible modus operandi 

 in such conditions, and much further work on the subject is 

 necessary. 



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. 204). 

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

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

 on subcutaneous inoculation 400 times the dose originally fatal. 

 Fraser also found in the case of snake venom that rabbits could, 

 by being fed with the poison, be immunised against several times 

 the lethal dose of venom injected into the tissues. In such 

 cases some of the molecules which act as antigens apparently 

 pass through the intestinal wall unchanged. 



By feeding animals with dead cultures of bacteria or with 

 their separated toxins, a degree of immunity may in some cases 

 be gradually developed. But this method is so much less certain 

 in results, and so much more tedious than the others, that it has 

 obtained no practical applications. 



Active immunity of high degree developed by the methods de- 

 scribed may be regarded as specific, in the sense explained below 

 (p. 558). A certain degree of immunity, or rather of increased 

 general resistance of parts of the body (for example, the peri- 

 toneum), can, however, be produced by the injection of various 

 substances bouillon, blood serum, solution of nuclein, etc. 

 (Issaeff). Also increased resistance to one organism can be thus 

 produced by injections of another organism. Immunity of this 

 kind, however, never reaches a high degree. 



B. Passive Immunity. 



Action of the Serum of Highly Immunised Animals. 1. 



The serum of an animal A, treated by repeated and gradually 

 increased doses of a toxin of a particular microbe, may protect 

 an animal B against a certain amount of the same toxin when 



