THE RATIONALE OF ACQUIRED IMMUNITY 247 



acquired the power of resisting weak toxins, he is in a position of 

 advantage from which he can react to the stronger toxins and so 

 acquire immunity. In these instances, then, acquired immunity is 

 nothing other than a ' use-acquirement,' resulting from progressive 

 habituation. The antitoxins educate the phagocytes till they are 

 able to tolerate the strongest toxins and destroy the microbes 

 producing them. 



413. That habituation plays a great part in the acquirement 

 of immunity is evident from the fact that, if we begin with small 

 but increasing doses, we may eventually inject very large doses 

 of diphtheria toxin into a horse, without making him ill, though his 

 serum becomes poisonous to other animals, which, of course, it 

 would not be were his immunity due to chemical neutralization. 

 So also, by beginning with small doses a certain degree of 

 immunity against morphia, nicotine, and arsenic may be achieved. 

 It is evident, therefore, that immunity may be secured with com- 

 parative ease in two ways, by giving weakened toxins, or by giving 

 small but increasing doses of unaltered toxins. The chemical 

 composition of morphia and nicotine is comparatively simple. 

 These substances, therefore, cannot be altered without destroying 

 them as poisons. Arsenic is a still more simple substance. But 

 the composition of the microbic toxins, that of such proteid 

 vegetable poisons as abrin and ricin, and that of such animal 

 poisons as snake venom is very complex. Therefore they may 

 be altered in such ways as to leave them still poisonous but 

 weakened in various degrees. If food be swallowed, it is digested 

 by the pepsin and the other ferments secreted by the alimentary 

 tract and passes into the system in a somewhat altered condition. 

 Similarly if snake poison be swallowed, it is digested and some- 

 what altered so that it enters the blood in an attenuated state and 

 educates the system against virulent venom injected under the 

 skin. 1 If the poison gland be swallowed, it is also digested. 

 Now microbes, during recovery from disease or if enclosed in a 

 capillary tube, perish though not in contact with phagocytes. 



present in the infected person in numbers sufficient to cause disease, that is, they 

 are present in such numbers that the phagocytes cannot overcome them ; 

 and their numbers continually increase. According to this hypothesis, then, if 

 we wish to cure a disease due to an increasing number of microbes, we must add 

 to the multitude from extraneous sources. 



1 Digestion in gastric juice of spinal cords infected with rabies, has also been 

 practised, especially in Italy as a means of attenuating the virus of that disease. 

 If a series of injections of this digested material be made into sheep an antitoxic 

 serum may be obtained. 



