LYSINS, AGGLUTININS, PRECIPITINS, ETC. 235 



munization, 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 various 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 de- 

 termined by agglutination tests do not always correspond to classi- 

 fications depending upon morphological and cultural characteristics. 



An interesting phenomenon of great practical importance, which 

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

 encoimtered in routine agglutination tests, is the frequent failure of a 

 strongly agglutinating serum to produce agglutination if used in concen- 

 tration, while in dilutions it produces a characteristic reaction. 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 agglu- 

 tinogen without causing agglutination. 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 them- 

 selves, and are, therefore, termed "proagglutinoids." In strongly 

 agglutinating sera these proagglutinoids may be present in considerable 

 quantities and prevent 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 hemagglutinins, or 

 substances which clump together red blood cells, are similarly formed. 



PRECIPITINS 



(Coagulins) 



In the year 1897, R. Kraus,' of Vienna, demonstrated that the sera 

 of animals inmiunized against B. pestis, B. typhosus, and Vibrio 

 choleras, when mixed with the clear filtrate of bouillon cultures of the 

 respective organisms, gave rise to macroscopically visible precipitates. 

 The precipitates occurred only when filtrate and immune serum were 

 homologous, that is to say, the animal from which the senun was ob- 

 tained was immunized by the same species of microorganism as that 

 used in the test; and for this reason Kraus spoke of them as "specific 



1 Kram, Wien. klin. Woch., 1897. 



