40 THE EXOTOXINS 



to diphtheria toxin (e.g., in the production of diphtheria antitoxin) 

 will be just as susceptible to tetanus toxin as a normal animal. 

 In a very few cases the law does not hold. The only well- 

 authenticated example of this sort is the antagonism which 

 animals display to anthrax after injection with the products of 

 B. pyocyaneus. 



These preliminary considerations will serve to show the more 

 important criteria by which the nature of a bacterial product may 

 be determined, and its nature as a true toxin established. 



These toxins were soon found to fall into two main groups the 

 extracellular or soluble toxins, or, as we shall call them, the exo- 

 toxins, and the intracellular insoluble toxins, or endotoxins. We 

 shall consider these substances in turn. 



THE EXOTOXINS. 



The exotoxins are substances which are given off in a free state 

 when the bacteria are grown in a suitable culture medium outside 

 the body, and can usually be separated by simple filtration (through 

 a Pasteur or Berkefeld filter) from the organisms which produce 

 them. We may consider them provisionally as the specific secre- 

 tions or excretions of the bacteria. They are not formed by all 

 pathogenic bacteria that is, in the present state of bacteriological 

 science no suitable culture media have been found in which certain 

 organisms will produce a soluble toxin. The three most impor- 

 tant organisms which do so are the B. tetani, B. diphtheria, 

 and the B. botulismiis. These toxins, the first two especially, 

 are substances of the greatest interest, since they have been 

 submitted to a most profound examination, and our knowledge 

 of the structure of bacterial toxins, of their action on the body, 

 and of the production of immunity thereto, is based almost entirely 

 on the results thus obtained. In addition to these, there are sub- 

 stances which are much less toxic if, indeed, toxic at all and 

 which fail to fulfil our definitions of a specific toxin, since an 

 animal which has been immunized thereto is not necessarily 

 immune to the organism, but which have many points in common 

 with the true toxins, and will be considered in this connection. 

 These are the bacterial cytolysins and haemolysins, * substances 



1 Haemolysis, or the liberation of haemoglobin from red blood-corpuscles, 

 may be brought about by a variety of agents, which fall under three main 

 headings : (i) Simple chemical substances, such as distilled water, ether, 

 acids, etc., which act osmotically, or by a direct solution of the stroma of 



