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 

 encountered 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. 



The theoretical considerations concerning the nature of agglutinins 

 are discussed below, together with a similar section on the precipitins. 



PRECIPITINS 



R. Kraus, 1 of Vienna, demonstrated that the sera of animals im- 

 munized against B. pestis, B. typhosus, and Vibrio cholerse, when 

 mixed with the clear filtrate of bouillon cultures of the respective or- 

 ganisms, produce macroscopically visible precipitates. These precipi- 

 tates occurred only when filtrate and immune serum were homologous, 

 i. e., when the animal from which the serum had been obtained had been 

 immunized by the same species of microorganism as that which was 

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



1 Kraus, Wien. klin. Woch., 1897. 



