SENSITIZING ANTIBODIES 289 



in a group agglutination, to the agglutinating serum. In this way, 

 specific and partial agglutinins can be separately removed from the 

 immune serum by absorption each by its corresponding agglutino- 

 gen. In such experiments all agglutinins will be removed by the 

 organisms used for immunization, a partial removal only resulting 

 from the addition of allied strains. This method has thrown much 

 light upon the intimate relations existing between members of vari- 

 ous' bacterial species, and has been particularly valuable in the study 

 of the typhoid-colon-dysentery group. It is important to mention, 

 however, that "groups" as determined by agglutination tests do 

 not always correspond to classifications depending upon mor- 

 phological and cultural characteristics. 



An interesting phenomenon of great practical importance, which 

 has been noticed by a number of observers, and which may often 

 be encountered in routine agglutination tests, is the frequent failure 

 of a strongly agglutinating serum to produce agglutination if used 

 in concentration, while in dilutions it produces a characteristic re- 

 action. This has been explained theoretically by what is known 

 as the "proagglutinoid zone." It is assumed that agglutinins may 

 deteriorate as do toxins and be converted into substances which are 

 capable of combining with agglutinogen without causing agglutina- 

 tion. Such substances, as we will see in discussing Ehrlich's views 

 on the structure of agglutinins, may have a stronger affinity for 

 agglutinogen than the agglutinins themselves, and are, therefore, 

 termed "proagglutlnoids." In strongly agglutinating sera these 

 proagglutinoids may be present in considerable quantities and pre- 

 vent the combination of agglutinin with agglutinogen. In dilution, 

 this proagglutinoid action would naturally become weaker and of 

 no actual significance in obscuring the reaction. 



Agglutination, like other immune phenomena, is a manifestation 

 of broad biological laws and not limited to bacteria. Thus, as 

 hemolysins are produced by the injection of red blood cells, so 

 hem agglutinins, or substances which clump together red blood cells, 

 are similarly formed. 



The theoretical considerations concerning the nature of agglu- 

 tinins are discussed below, together with a similar section on the 

 precipitins. 



Precipitation. R. Kraus, 31 of Vienna, demonstrated that the sera 

 of animals immunized aaginst B. pestis, B. typhosus, and Vibrio 



31 Kraus, Wien. klin. Woch., 1897, 



