520 IMMUNITY 



b. coli, infections of joints by the gonococcus, and in many cases 

 considerable 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. 199). 

 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 feeding with the poison, be immunised 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 

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

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

 injected along with the latter, or a short time before it. As 

 might be expected, it has less effect when injected some time 

 afterwards, but even then within certain limits it has a degree 

 of curative or palliative power. Seeing that the serum of animal 

 A appears to neutralise the toxin, the term antitoxic has been 

 applied to it. 



2. The serum of an animal A, highly immunised against a 

 bacterium by repeated and gradually increasing doses of the 

 organism, may protect an animal B against an infection by 

 the living organism when injected under conditions similar to the 

 above. This serum is therefore antimicrobic, or antibacterial, 



