IMMUNITY AND SUSCEPTIBILITY 695 



question. These experiments may be made in vitro. When toxins 

 and antitoxins are combined in proper proportion and incubated 

 together a non-toxic molecule is produced which when injected into a 

 susceptible animal will produce no effect. It is, of course, necessary 

 in this connection to inject the animal with a minimum lethal dose of 

 the toxin in question as a control. If no natural antitoxins are present 

 in the serum of the animal in question, the animal experimentally 

 injected with the combined toxin and serum will die as a result of the 

 non-combination of the toxin. In this way natural antitoxins may 

 be tested. Natural antitoxins for diphtheria have been detected in 

 the blood serum of about fifty per cent of normal humans and in about 

 thirty per cent of horses. However, their occurrence in other animals 

 for this specific bacterium and for other species is comparatively rare, 

 and the explanation of the fact that certain animals are immune to 

 toxins must be found elsewhere. It has been shown for example that 

 the frog is immune to tetanus toxin, and that, when this animal is 

 injected with this toxin, a large part of the toxin remains unchanged 

 in the circulation for a variable period of time and may be later drawn 

 off in the blood serum, producing a toxic effect when injected 

 into a susceptible animal. There are no natural antitoxins present in 

 the blood serum of the frog and it has been found that the immunity 

 of this animal is due to the fact that there are no cells in the body 

 possessing the necessary side-chains (open valencies) for chemical 

 combination with the toxin and the subsequent intoxication of the 

 cells does not result. It seems that the best explanation of the fact 

 that certain animals are immune to toxins is found in the fact that 

 there are no chemical substances in the cells with which toxin can 

 combine. It is probably not true that natural antitoxins explain all 

 the phenomena in this connection. 



Natural Antibacterial Substances. Natural antibacterial substances 

 are present in the blood serum and body fluids of a large number of 

 animals. In order to demonstrate the presence of the natural anti- 

 bacterial substances it is necessary to inject the experimental animal 

 with a carefully washed culture of the bacteria in question. If the 

 animal remains uninfected, two possibilities present themselves: First, 

 the presence of natural antitoxins; and second, the presence of anti- 

 bacterial substances. It is necessary, of course, to have excluded the 

 possibility of natural antitoxins, it having been demonstrated that 



