704 MICROBIOLOGY OF DISEASES OF. MAN AND DOMESTIC ANIMALS 



to the L dose of the toxin* and injecting the resulting mixture into 

 test animals (guinea-pigs), there is not a corresponding decrease in the 

 toxicity as would be expected. The toxin is made up of various parts. 

 The part just mentioned has a great affinity for the antitoxin but is not 

 really toxic. Such parts of the toxin molecule are called protoxoids. 

 The protoxoids compose about one-fourth of the amount of toxin 

 necessary to saturate one immunity unit. After the one-fourth 

 antitoxin is added to the L dose of toxin the mixtures of toxin-anti- 

 toxin become less toxic for the experimental animals down to the point 

 where approximately three-fourths of the amount of toxin necessary 

 to saturate one unit of antitoxin has been used (three-fourths of L 

 dose). This fraction is true toxin. The toxicity of the mixture does 

 not decline from this point when antitoxin is added up to one im- 

 munity unit, and it has been demonstrated by Ehrlich and others that 

 this is due ^o another part of the toxin molecule which has a lesser 

 affinity for the antitoxin than the true toxin itself and the protoxoid. 

 This part of the molecule is called an epitoxoid, 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, true toxoid or toxon. The toxon is in certain instances 

 slightly toxic and is supposed by some to be a secondary toxin and in 

 certain diseases such as diphtheria, this substance has a weak affinity 

 for antitoxin and is a possible cause of diphtheritic paralysis. 



Antitoxins may be prepared for all the bacteria producing soluble 

 toxins, such as Bact. diphtheria, B. tetani, B. botulinus and Ps. pyo- 

 cyanea. Antitoxic substances may also be made for some of the products 

 of other bacteria such as the Strept. pyogenes but these differ from true 

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

 plant cells, such as abrin, recin, 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 disintegrating bacteria, those 

 disintegrating and liberating the hemoglobin of erythrocytes (red 



* The amount which just neutralizes one unit of standard antitoxin (United States Public 

 Health Service). 



