IMMUNITY 211 



tetanus toxin and antitoxin which is neutral for the guinea-pig 

 is fatal to the mouse ; it is impossible to avoid the conviction 

 that the body, that of the guinea-pig or of the mouse in this 

 example, counts for something in the phenomena. The same 

 ideas were maintained by Roux at the time of the discovery of 

 serotherapy. 



But, on the other hand, the action of antitoxin on toxin 

 seemed to be a neutralization, and to behave both in vitro and 

 in vivo like a chemical reaction ; and in practice it was found 

 necessary for medical purposes to titrate the sera in this way 

 to measure their activity ; the ideas of Buchner and of Roux 

 were then laid aside, only to be rediscovered later; the 

 biological phenomena were subordinated as much as possible 

 to quantitative studies, and the endeavour was made to 

 represent the action of antitoxins on toxins as a chemical 

 reaction. 



The Side-chain Theory. — The best known chemical 

 theory for the action of antitoxins and antibodies in general 

 is that of Ehrlich, which is currently known under the name 

 of the "side-chain theory." The primary idea of its author 

 was to find in the facts mutual relations as much as possible 

 fixed and independent of the body, and to eliminate all 

 " vitalism '' in favour of exact quantitative work. To begin 

 with, he adopted the method of in vitro experiment. The 

 nature of the tetanus and diphtheria toxins had been rendered 

 much clearer by the study of other toxins more easy to work 

 with, such as haemolysins, agglutinins, ferments, and anti- 

 ferments (ricin, abrin, rennin, and antirennin, etc.). Pre- 

 liminary experiments in vitro showed the general applicability 

 of the same laws. Originally Ehrlich believed that the curative 

 and preventive action in vivo was equivalent to the neutralising 

 action towards the diphtheria toxin in vitro, a belief which later 

 experiments were to disturb. 



The second step was to demonstrate that antitoxin does not 

 destroy toxin, but that the two bodies combine to form a com- 

 pound (neutral from the physiological point of view), just as an 

 acid and a base combine to form a salt. 



P 2 



