550 MICROBIOLOGY OF THE DISEASES OF MAN AND ANIMALS. 



animal cells. The question arises as to whether these animals are immune 

 to the toxins on account of the presence in their bodies of natural antitox- 

 ins or other substances. If antitoxin is present it can be detected by 

 experiments made by drawing the serum of the animal and combining it 

 in varying proportions with the toxin in 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 pro- 

 duce no effect. It is, of course, necessary 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 50 per cent of normal 

 individuals and about 30 per cent of horses. However, their occurrence 

 in other animals for this specific bacterium and for other species is com- 

 paratively 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 serum, and will produce a toxic effect when 

 injected into a susceptible animal. There are no natural antitoxins 

 present in the blood serum of the frog. 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, consequently the intoxication of the cells 

 does not result. It seems that the best explanation that certain animals 

 are immune to toxins is ascribable to the fact that the-re are no chemical 

 substances in the cells with which toxin can combine. It is probably 

 not true then- 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 antibac- 

 terial substances it is necessary to inject the experimental animal with a 

 carefully washed culture of the bacteria in question. If the animal re- 



