ATREPTIC IMMUNITY. 127 



Investigations have not advanced far toward the 

 positive determination of the correctness of the 

 hypothesis. 



In Ehrlich's conception of "atreptic" immunity, 

 the possibility is entertained that micro-organisms 

 sometimes may not be able to grow in a host 

 because the nutritive conditions are not suitable. 

 It probably would be granted that proper nutritive 

 substances are present in the host, but it is assumed 

 that they may be so intimately and firmly bound 

 to other tissue constituents that they are not avail- 

 able to the micro-organisms as food. Virulent 

 micro-organisms find the nutritive substances 

 already available, or, if this is not the condition, 

 the injury which they effect on the tissues in the 

 first instance results in a splitting of the con- 

 stituents so that the food substance (rather a spe- 

 cific food substance) becomes available. Avirulent 

 organisms not having the power to produce this 

 splitting result do not proliferate to any great 

 degree and soon become the prey of the protective 

 agencies, such as the leucocytes and bacteriolysins. 

 With this theory in mind, one recurs to the 

 "exhaustion" theory of immunity at one time 

 entertained by Pasteur, and cited in a previous 

 chapter. This difference appears, however, that in 

 Pasteur's theory an entire absence of suitable food 

 was assumed, whereas in the atreptic theory the 

 food substance may be present but not available for 

 the use of the organisms. The atreptic theory is 

 put forward as being a possible factor in resistance 

 to some infections by some hosts, not, of course, as 

 a theory intended to supplant other general 

 theories of immunity. 



