464 IMMUNITY 



only an aggravation of symptoms is to be looked for (vide 

 pp. 194, 261). 



With regard to the details of the preparation of the vaccines, 

 an agar culture is taken, the growth removed into normal saline 

 and killed by steaming for a sufficient time say 1J hours. 

 The efficiency of the sterilisation is tested by inoculating tubes 

 of appropriate media. The strength of the emulsion is then 

 estimated by the method of counting dead bacteria described on 

 p. 67. The number of bacteria employed for a vaccination is 

 usually from 250,000,000 to 500,000,000, and in the details of 

 the measurement of this quantity and in its injection, every 

 aseptic precaution must, of course, be adopted. Such vaccines 

 have been used extensively in the treatment of acne, boils, 

 sycosis, infections of the genito-urinary tract by the b. coli, 

 infections of joints by the gonococcus, and in many cases con- 

 siderable success has followed the treatment. 



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. 169). 

 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 

 be immunised, by feeding with the poison, 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 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 

 described may be regarded as specific, that is, is exerted only 

 towards the organism or toxin by means of which it has been 

 produced. 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 



