AGGLUTININS AGGLUTINOIDS AND PROAGGLUTINOIDS 147 



is known as a " proagglutinoid" reaction; it is attributed by Ehrlich 

 to the presence of " agglutinoids" in the serum side-chains of the 

 second order which have lost their agglutinophore group, but still 

 retain their combining group (haptophore group). These " agglu- 

 tinoids," which are deteriorated agglutinins, have a greater affinity 

 for the agglutinogen of the bacteria than have the unchanged agglu- 

 tinins, and consequently prevent the latter from becoming attached 

 to the organisms. If the serum is diluted a point is reached where 

 the agglutinoids are numerically too few to interfere with the action 

 of the agglutinins, which usually far outnumber the agglutinoids. As 

 the serum is more and more diluted a point is eventually reached 

 where the content of agglutinin is insufficient to react with the bacteria. 

 If, however, bacteria are cultured in this dilute serum, they frequently 

 develop into long, thread-like, interwoven filaments, the so-called 

 "thread-reaction" of Pfaundler. It is obvious that the maximum 

 dilution at which a serum will agglutinate bacteria depends somewhat 

 upon the number of organisms; there is, in other words, a relation 

 between the amount of agglutinin in the serum and agglutinogen in 

 the bacteria. 



Non-agglutinable Bacteria. Occasionally strains of bacteria, as 

 B. typhosus, freshly isolated from the body, may not agglutinate 

 with the specific serum. This resistance to agglutination is supposed 

 to result from some unknown change in the agglutinogen of the bac- 

 terium during its development in the body. A similar loss of agglu- 

 tinability may be experimentally brought about by growing the 

 bacteria in gradually increasing concentrations of specific agglutinat- 

 ing serum outside the body. This inagglutinability is usually lost 

 after a few days' development on artificial media; the organisms will 

 then clump in a characteristic manner in a serum that originally was 

 ineffective. 



The Reaction of Agglutination. The practical value of the reaction 

 of agglutination depends upon the visible clumping or agglomeration 

 of a suspension of bacteria in a fluid medium containing some neutral 

 salt, when a relatively small amount of immune serum specific for 

 the organism is added to it. The reaction may be expressed thus: 



Organism (Agglutinogen) + Specific Serum (Specific Agglutinin) 

 = Agglutination. 



If a specific organism is added to an appropriate dilution of unknown 

 serum with proper precautions, and characteristic clumping takes 

 place, or if a known specific serum is added with suitable precautions 



