IMMUNITY AND SUSCEPTIBILITy 717 



tinate. Occasionally in some fresh serums substances are found which 

 have a greater affinity for the agglutinogen of the bacteria than the 

 agglutinins have. Such substances are designated as proagglutinoids 

 and are in this respect similar to protoxoids. 



The Stages of Agglutination. — There are two distinct stages of the 

 agglutination reaction. Neither of these stages can take place unless 

 some salts or electrolytes are present. Sodium chloride is the common 

 salt present. The first phase of the agglutination reaction is a union 

 between the agglutinin and the agglutinogen of the bacteria. The 

 second phase is the actual clumping of the bacteria. It is supposed that 

 in this last phase the zymogenic group of the agglutinin is acting. 

 In the first phase the haptophile group of the agglutinin is combined 

 with the haptophore group of the agglutinogen. 



There are sbme bacteria that cannot be agglutinated, as for example, 

 Bact. pneumonicB of Friedlander, and in rare instances B. typhosus 

 cannot be agglutinated. It is possible, for example, to grow B. typhosus 

 at a temperature of 42° and cause it to lose its power of producing 

 agglutinins. Bacteria may also be modified chemically so that they 

 will lose the power to produce agglutinins. 



Agglutinins bear no relationship to bactericidal substances, anti- 

 toxins, opsonins or any of the other antibodies. They are both of use 

 in the determination of species of bacteria when a known agglutinating 

 serum is used, and they are also of use in determining the cause of 

 infections where a known culture or agglutinogenic substance is used. 

 The agglutination reaction is used in the diagnosis of typhoid fever, 

 paratyphoid fever, glanders and dysentery. 



Hemo agglutinins. — Agglutinating substances are sometimes pro- 

 duced for red blood corpuscles when these cells are used in the immu- 

 nization of an animal. Such agglutinins when combined with the 

 corpuscles produce a clumping which is known as hemoagglutina- 

 tion. The mechanism of the reaction is the same as that of bacterial 

 agglutinins. It is possible that hemoagglutination is one impor- 

 tant factor in the production of agglutination thrombi in certain 

 infectious diseases such as typhoid fever. 



Precipitins. — Another group of substances, which are antibodies, 

 is produced through the processes of immunization which have not 

 been definitely connected with the phenomena of immunity. These 

 substances are known as the precipitins. Precipitins may be produced 



