THE PROTECTIVE SUBSTANCES OF THE BLOOD. 367 



There is one fact, however, by which the transformation hypothesis 

 is especially refuted, namely, that antitoxins can occur in the blood 

 of normal individuals. Thus diphtheria antitoxin is found in the 

 blood of horses in about 20-30% f the animals examined, although 

 diphtheria infection is surely a rare exception with these animals. 

 Horse serum furthermore contains antibodies against one of the poisons 

 produced by tetanus bacilli, tetanolysin, but not against the tetanizing 

 poison of the same bacilli, the tetanospasmin, although the immune 

 serum artificially produced contains both antibodies. 



Just these observations, which can easily be extended, show that 

 even the normal organism can produce true antitoxins without the 

 intervention of the corresponding bacterial substances. Hence these 

 antibodies cannot be transformation products of the poisons injected, 

 but are products of normal cell activity. The explanation especially 

 of these normal processes constitutes one of the chief points in the side- 

 chain theory. 



This theory is based primarily on a thorough analysis of the 

 relations between toxin and antitoxin. It was found, by means of 

 test-tube experiments with ricin and related bodies which act on red 

 blood-cells, that it was extremely probable that toxin and antitoxin 

 act chemically directly on each other, forming a new innocuous com- 

 bination. It was now necessary to study the neutralization of these 

 two substances in all directions in great detail. For this purpose I 

 chose diphtheria toxin and antitoxin, because the guinea-pig organism 

 furnishes such a uniform test object for this poison that exact quan- 

 titative determinations, such as are used in physics and chemistry, 

 are attainable in animal experiments. The limit of error in the titra- 

 tion of diphtheria serum titrations is not more than 1%, surely an 

 astonishing result if we consider that we are dealing with substances 

 which chemically as } T et are entirely unknown. 



The results which I obtained in the earlier years of my investiga- 

 tions were really very discouraging, for they seemed to present an 

 insurmountable obstacle for the chemical conception. In chemical 

 processes when two substances unite to form a third substance, 

 in accordance with the laws of stoichiometry, we must insist that these 

 components act on one another in definite equivalent proportions. 

 In the action of diphtheria antitoxin or toxin, however, this law 

 seemed to be utterly disregarded. Thus in twelve different toxic 

 bouillons I first determined the quantity which was neutralized by a 

 constant amount of antitoxin; in certain instances by the official 



