AGGLUTININS AND PRECIPITINS 89 



It will be seen from these protocols that absorption by the major agglu- 

 tinogen, bacillus typhosus, removes both the major and minor agglutinins, 

 and that absorption by the minor agglutinogen removes only the minor agglu- 

 tinin, although it is true that even though the titer of the major agglutinin is 

 not reduced it may agglutinate in smaller clumps. 



Inhibition Zones. It is sometimes found that in powerful agglu- 

 tinins there is an " inhibition zone " in the more concentrated dilu- 

 tions. Thus a serum may agglutinate as follows : 



Tube Serum dilution Result 



^ I I-IO 



2 I-IOO + 



3 1-1,000 +-H- 



4 1-2,000 -f-H- 



5 1-4,000 -} f- 



6 1-6,000 + 



7 1-8,000 



This phenomenon is somewhat more frequently observed in sera 

 that have been preserved for a considerable time in the moist state. 

 If a serum with a titer of i-iooo, which originally showed agglu- 

 tination in all dilutions up to 1000, is preserved and after several 

 months titrated again, it may fail to agglutinate in i-io, may 

 agglutinate only weakly in i-ioo, and completely in 1-500. If the 

 tube containing i-io dilution is centrifuged, the supernatant fluid 

 drawn off, the bacteria again suspended and placed with the serum 

 in dilution of 1-500, there is no agglutination. The same is true if 

 these treated organisms are placed in contact with a fresh agglu- 

 tinating serum. The same phenomenon is obtained if the serum 

 first used is a fresh one of high titer with an inhibition zone, 

 and the bacteria are removed from the low dilutions in which they 

 have failed to agglutinate. The bacteria have become inagglutin- 

 able by treatment with the serum in these concentrations. Simi- 

 larly, heating an agglutinating serum to 65 to 70 C. destroys its 

 agglutinating properties, but if it is added to bacteria they become 

 inagglutinable when treated with fresh active serum. This phe- 

 nomenon is strictly specific and operates only in the presence of the 

 homologous organism. This peculiar character of agglutinins has 

 been closely linked with the Ehrlich conception of immune bodies 

 and is explained as due to the presence in sufficient concentration of 

 " agglutinoids." The term agglutinoid is applied to that part of the 

 agglutinin which has a specific binding affinity for the cell, but has 

 been deprived of the thermolabile and more easily destructible frac- 

 tion which has the power of producing clumping. This explanation 

 will be discussed more in detail in the general discussion 

 of agglutinins. 



The influence of heat on agglutination has been studied exten- 

 sively. As has been indicated, heat will destroy agglutinins, but 

 certain agglutinins are destroyed by degrees of heat which fail to 

 destroy others. Most agglutinating sera are rendered inactive at 

 60 to 65 C., but anti-plague agglutinin is destroyed at 56 C, 



