IMMUNITY AND SUSCEPTIBILITY. 559 



called an epUoxoid, true toxoid or toxon. The toxin molecule necessary to 

 saturate one unit of antitoxin is, therefore, made up of one-fourth protoxoid, 

 one-half true toxin, and one-fourth epitoxoid, toxoid, or toxon. The toxon 

 is in certain instances slightly toxic and is supposed by some to be a second- 

 ary toxin and in certain diseases, such as diphtheria, this substance which 

 has a weak affinity for antitoxin is supposed to be the cause of diph- 

 theritic paralysis. 



Antitoxins may be prepared for all the bacteria producing soluble 

 toxins, such as Bad. diphtheria, B. tetani, B. botulinus, and Ps. pyocyanea. 

 Antitoxic substances may also be made for some of the products of other 

 bacteria such as the Strept. pyogenes, but these differ from true anti- 

 toxins. Antitoxins may also be prepared for the toxins of certain plant 

 cells such as abrin, ricin, crotin, and for the toxins of animals, such as snake 

 venom and spider poison. These substances are in the main similar to 

 those produced by bacteria, although in certain characteristics they differ 

 materially. 



LYSINS AND BACTERICIDAL SUBSTANCES. Under the lysins will be 

 discussed those substances occurring in normal and immune sera which 

 have the power of destroying and dissolving bacteria, those dissolving or 

 liberating the hemoglobin of erythrocytes and those substances which 

 have a lytic action on various body cells. The substances which act on 

 the bacteria are called bacteriolysins, those acting on erythrocytes are called 

 hemolysins and those acting on cells in general are called cytolysins. 

 The mechanism of these lytic processes is quite complex. There are 

 certain substances which kill or seriously injure bacteria and body cells 

 and do not actually dissolve them. Such chemical bodies are designated 

 respectively as bactericidal substances and cytotoxins. 



The first observations in regard to bactericidal and bacteriolytic sub- 

 stances were made by Nuttall and later by Buchner. Buchner noted these 

 substances in normal sera and other body fluids and named them alexins 

 (Gr. to guard) . He assumed that they were the important substances con- 

 cerned in the immunity of the body. This is not necessarily true, as cer- 

 tain blood sera are frequently highly bactericidal and the individual is rela- 

 tively susceptible. This is true of human blood serum and B. typhosus. 

 Furthermore, in certain instances the animal is immune to the disease and 

 the serum is not in any sense bactericidal. This is the case with the dog 

 and Bad. anthracis. 



Pfeiffer a number of years ago observed that when Msp. comma, of 



