442 IMMUNITY. 



protect an animal against the living organisms has already 

 been explained. We know little regarding the mode and 

 seat of formation of the antitoxines, though they are, 

 doubtless, the products of cellular activity induced by the 

 toxine. It has been shown that a horse, after its serum 

 has reached a high antitoxic power, may be repeatedly 

 bled and the serum still maintain this power little dimin- 

 ished, though no more toxine has been injected. In this 

 case it would appear that the formation of antitoxine con- 

 tinued after the stimulus supplied by the toxine had been 

 removed. 



Antitoxine, when present in the serum, leaves the body 

 by the various secretions, and in these it has been found 

 though in much less concentration than in the blood. It 

 is present in the milk, and a certain degree of immunity can 

 be conferred on animals by feeding them with such milk, 

 as has been shown by Ehrlich, Klemperer, and others. 

 Klemperer also found traces of antitoxine in the yolk of 

 eggs of hens whose serum contained antitoxine. 



2. Antimicrobic Serum. We have already stated that 

 an antitoxic serum, so far as is known, also protects from 

 an invasion of the corresponding organism, though it has 

 no actually bactericidal properties. In such a case the 

 toxines of the organisms are rendered non-effective by the 

 antitoxic serum, and the organisms themselves may then be 

 destroyed by the means normally possessed by the tissues. 

 A serum which protects in a high degree from such microbic 

 invasion can, however, be obtained also by injection of 

 gradually increasing doses of living cultures. A serum 

 prepared by the latter method has often little or no antitoxic 

 power, and appears to exercise its beneficial effect by lead- 

 ing indirectly to the death of the organisms. Hence it is 

 called antimicrobic. 



The stages in preparation of antimicrobic sera corre- 

 spond to those in the case of antitoxic sera, but living, or, 

 in the early stages, dead cultures are used instead of the 

 toxine separated by filtration, and in order to obtain a 

 serum of high antimicrobic power, a very virulent culture 



