196 INFECTION AND IMMUNITY. 



lich and his followers continue to uphold the toxon as 

 an independent toxic substance, and have published 

 additional experiments to support their position. 



Let one now add still smaller amounts of the 

 antitoxin unit to the 200 binding units of the 

 toxin. When 149/200 are added it is found that 

 a certain amount of true toxin remains free, the 

 quantity which is unbound being in direct propor- 

 tion to the amount of antitoxin withheld. Conse- 

 quently when but 50/200 antitoxin unit is added 

 the amount of free toxin corresponds to 100 bind- 

 ing units. If true toxin only remained it could 

 then be said that the constitution of this toxin is : 

 toxin 150 and toxon 50. However, it may be 

 found that as 49/200, 48/200, etc., to 0/200 anti- 

 toxin unit are added, no increase of free toxin is 

 found, although the antitoxin added has been 

 bound. In this case, the 50 binding units of toxin 

 which have the greatest affinity for antitoxin are 

 non-toxic; i. e., they are toxoids, and since they 

 have the maximum affinity for antitoxin they are 

 called protoxoids. 



It has been assumed also that a toxoid may 

 exist which has an affinity for antitoxin exactly 

 equaling that which toxin possesses; this, as yet 

 purely hypothetical constituent, bears the name of 

 syntoxoid. 



Figure 1 is a graphic representation of the 

 toxin just described (Madsen). Probably no two 

 toxins have the same constitution. The toxon 

 zone, for example, could well be much larger in 

 one diphtheria toxin than in another. 

 Proto-, Deu- Refinements in experimentation show that even 

 the true toxin is not uniform in its virulence and 

 its affinity for antitoxin. Accordingly a proto- 



