2 • SCIENCE PROGRESS. 



from the assumed analogy between fermentative processes 

 and infectious diseases, and stated that, just as in the alco- 

 holic fermentation the yeast cells, that is the specific microbe, 

 require for their life and action the presence of sugar, so also 

 in the infectious diseases the presence in the body of special 

 substances required by the particular microbe is an essential 

 element : without them the microbe cannot live and thrive, 

 and cannot produce the disease. 



This theory further assumed, as a necessary corollary, 

 that during the progress of the disease the fermentative 

 substance becomes consumed, and as soon as this stage is 

 reached the disease comes to an end ; as long as the 

 exhaustion of the fermentable substance lasts the body 

 is unsusceptible to further attack. This explanation of 

 acquired immunity is generally spoken of as the exhaustion 

 theory. We know now that this theory is not borne out 

 by direct evidence, moreover it is definitely contradicted by 

 fact. The infectious diseases are not comparable to fer- 

 mentative changes, such as the alcoholic or acetic fermen- 

 tation, and we also know that acquired immunity is not due 

 to exhaustion and removal by the specific microbe of some 

 necessary ingredient from the tissues during the first attack. 



Klebs first formulated a further theory, which is known 

 as the antitoxin or antidote theory ; this theory explained 

 acquired immunity by saying that during the life and multi- 

 plication of the microbes within the infected body — that is, 

 during the attack of the disease — these microbes produce 

 chemical substances which are poisonous to the microbes 

 themselves and their products, so that by the accumulation 

 of these antitoxins in the infected body the disease comes 

 to an end, and reinfection by the same microbes is pre- 

 vented as long as these antitoxins remain in the body. 1 

 We shall presently see that recent experimental research 

 has shown that this explanation of acquired immunity is the 

 correct one, although it ought to be stated that the view 



1 In my book, Micro-organism and Disease, 3rd edition, 1886, I have 

 in chapter xx. fully considered and supported this theory, and have pointed 

 out that natural immunity and acquired immunity are carefully to be dis- 

 tinguished from one another, a conclusion to which also Buchner arrived 

 within recent years. 



