260 STUDIES IN IMMUNITY. 



substances. Many investigators have endeavored to solve this 

 problem, but, of necessity, with only partial success. As the molec- 

 ular constitution of these substances is a mystery and their chemi- 

 cal nature unknown, their interaction naturally could not be so 

 exactly and clearly demonstrated as is possible in dealing with the 

 well-known substances with which chemists work. 



We must be content, then, for the present with outlining the 

 general characters of the reaction, in describing its appearance and 

 in endeavoring to determine the laws to which it is subject. The 

 first question to answer obviously is : does antitoxin unite with toxin 

 in definite proportions, as does a monobasic acid with an alkali, 

 in which case the neutralized product has a fixed and invariable 

 composition; or does one of the substances unite with the other 

 in varying amounts? If this second supposition is correct, it is 

 evident that the combination formed by union of the two substances 

 will not always be the same or endowed with the same characteris- 

 tics, but will vary in composition according to the relative propor- 

 tions of the two reacting substances. For instance, a mixture of 

 equal volumes of toxin and antitoxin would produce a different 

 substance than a mixture of one part of toxin with two of antitoxin. 

 The resulting substance would vary with the respective doses of 

 the two components employed : all would contain the same elements, 

 toxin and antitoxin, but would differ in that one substance would 

 be more or less saturated by the other. According to this hypothe- 

 sis, the reaction of toxin and antitoxin would resemble, at least in 

 regard to the variation of proportions, the reaction of iodine on 

 starch. Starch, as we know, absorbs varying amounts of iodine 

 and correspondingly varies in the intensity of its blue color; for 

 this reason chemists regard this reaction as belonging to the phe- 

 nomena of dyeing. Dyed substances take widely varying amounts 

 of the dye. 



Let us consider for a moment this comparison with dyeing phe- 

 nomena, a comparison that we have previously made use of, and 

 it would seem inadvisedly, since it has led many who have read 

 our preceding articles to a thorough misconception of them. 



As we have determined,* the maximal amount of red blood cells 

 that a given dose of hemolytic serum can destroy varies in accord- 



* See p. 194. 



