BACTERIAL POISONS 35 



them from the "endotoxins." These characteristics they share with 

 a number of non-bacterial substances such as the vegetable poisons 

 ricin, crotin, and abrin, with animal poisons like snake venom and 

 spider poison (arachnolysin), and, in certain important respects, 

 with the substances spoken of as enzymes. 



Thus the bacterial true toxins are not biologically unique sub- 

 stances. Both in themselves and in regard to the reactions they 

 elicit when injected into the animal body, they share certain cardinal 

 properties with analogous substances derived from the higher plants 

 and from animals. And it is important to recognize at once that we 

 are dealing here, as in other phases of the study of bacterial immu- 

 nity, with broad biological laws, which find application not only in 

 bacteriology, but in general pathology and in the phenomena of pro- 

 tein metabolism in general. It so happens that these phenomena 

 have been studied and are most easily elucidated in connection with 

 bacteria. But their general significance must not be lost sight of. 



The cardinal characteristic which unites air of these substances 

 into a single well-defined biological group is their property of in- 

 ducing the formation of antitoxins when injected into animals. This 

 property is so important and its thorough comprehension so essential 

 that we may be permitted to digress briefly in order to make it clear. 



As we shall see, in subsequent chapters, all substances which lead 

 to the formation of specifically reacting antibodies in the treated 

 animal are spoken of as '^Imfigem* 7)r ""antibody-inducing sub- 

 stances." The class of "antigens" is a large one, including all 

 known proteins, and possibly some of the higher proteid split prod- 

 ucts, and protein-lipoid combinations, though the "antigenic" prop- 

 erties of the last two are still in controversy. But among this 

 large group of substances it is only the bacterial true toxins (exo- 

 toxins), obtained in broth filtrates of living cultures, together 

 with the vegetable poisons and other substances we have classified 

 with them above, which induce in the blood of the treated animal a 

 neutralizing antibody (antitoxin) which inhibits quantity for 

 quantity the activity of the injected toxin or vegetable or animal 

 poison. This property of eliciting the production of antitoxin in the 

 animal body alone separates these substances sharply from all other 

 antigens, toxic or otherwise, and, in this respect, they differ sharply 

 from the so-called "endotoxins" against which no antitoxins can be 

 produced. 



As an important secondary characteristic of this group of sub- 

 stances we may regard their chemically indefinable nature. In the 

 case of none of them have we any definite knowledge of chemical 

 constitution except in so far as it has been hitherto impossible to 

 separate them from the protein molecule. The intensive chemical 

 study of the toxins has universally resulted in failure to obtain a 

 protein-free product which has the characteristic toxic properties of 



