252 ACQUIRED IMMUNITY 



production and shedding of receptors occurs normally in the 

 presence of over much nutriment. Presumably, it does not, since 

 the free receptors would render the food incapable of being taken 

 up by the cells and thus over much nutriment would eventually 

 be equivalent to too little, and starvation would result. In any 

 case, the power of acquiring immunity is, according to Ehrlich's 

 theory, an accidental accompaniment of the power of assimilating 

 nutriment. 



420. On the other hand, inborn immunity the kind which 

 prevents an animal ever acquiring a disease is thought to depend 

 (i) on a lack of receptors which have an affinity for the toxins of 

 that particular disease, or (2) on the normal presence of free 

 receptors in the blood that is, on receptors which for some 

 reason are free in the serum, though disease has not stimulated 

 their super-production. Such non-bacterial poisons as morphia, 

 nicotine, arsenic and the like which are not of very complex 

 composition, are supposed to affect not the side-chains especially, 

 but the whole cell, which is thus poisoned or else acquires resisting 

 power against them in ways that differ from that in which it 

 acquires immunity against the proteid poisons. 



421. Obviously, the physiological part of Ehrlich's theory 

 attributes immunity to use-acquirement. It attempts to explain 

 up to a certain point how toleration of toxins arises ; that is, 

 Ehrlich supposes that toxins stimulate cells to the production of 

 receptors. But the central mystery of how it happens that the 

 stimulation has the effect alleged, remains unsolved. Instead of 

 direct digestion by enzymes, neutralization and subsequent dis- 

 integration (which must be due to some form of digestion), are 

 imagined. But, since it is admitted that toleration of poisons of 

 simple composition, such as morphine and nicotine, is not due to 

 an increased production of receptors but merely to increased 

 toleration by the cell itself, the necessity of postulating the 

 existence of the receptors is not very evident. No account is 

 taken of the evolution of the power of attack in bacteria nor of 

 resisting power in the species attacked. In fact the whole in- 

 dubitable truth of evolution is ignored. Disease is treated, not as 

 part of the normal environment as it really is, but as an accident 

 which is resisted by accident. Immunity, inborn and acquired, 

 is regarded, not as a product of protective evolution, but as a 

 fortunate ' fluke.' 



422. Some varieties of animals (e.g. white rats) are normally 

 immune to certain diseases to which allied varieties or species 



