JOHN H. NORTHROP 8oi 



decrease extending over a period of days. If the potential immediately after the addi- 

 tion of the electrolyte were slightly above the critical value, it is readily seen that the 

 flocculation might be caused by the second slow potential change. It is just in these 

 cases that the peculiarity manifests itself. When the potential is reduced at once to 

 zero, ordinary rapid coagulation occurs. 



SUMMARY 



The mechanism of agglutination just described makes it possible to analyze in 

 detail the picture of the agglutination process outlined at the beginning of this chap- 

 ter. In a suspension of bacteria the individual cells are kept apart by the repulsion due 

 to their like charges. If a substance is added which decreases this charge, the cells 

 approach one another more closely and finally become attached unless at the same 

 time the added substance destroys the cohesive force or "stickiness" of the cells. This 

 appears to be the case with concentrated solutions of electrolytes and most unsensi- 

 tized bacteria. If immune serum or other substances which form films on the bacteria 

 are added the bacteria take on more and more of the nature of the added substance, 

 and this may or may not lead to agglutination. If the potential of the bacteria was 

 originally different from that of the added substance under the same conditions, the 

 potential will be changed and will approach that of the added substance. If this is less 

 than the critical potential of that substance, agglutination will occur. If the potential 

 is greater than the critical potential of the substance forming the film, no agglutina- 

 tion will occur until electrolytes have been added to reduce the potential below the 

 critical value. If the potential on the cells happens to be the same as that of the sen- 

 sitizer, no change in potential will be noted but the suspension may still agglutinate 

 if this potential is less than the critical value for the sensitizer. The agglutination of 

 bacteria by their homologous antibody (agglutinin) differs from that of agglutination 

 by ordinary proteins or the agglutination of collodion particles by means of egg al- 

 bumin in that the antibody will form a film only on the bacteria used to produce it. 

 The specific part of the reaction, therefore, lies in the formation of the film. This 

 specificity is of the greatest theoretical and practical importance but does not differ, 

 in the writer's opinion, from the specificity of any chemical reaction. Quantitatively 

 every chemical reaction is specific, and the specificity of immune and enzyme reac- 

 tions differs only in degree from that of any other reaction. The problem of specificity 

 is one of reactions in general. It is possible with our present knowledge to predict 

 whether or not a given reaction will take place from considerations of the free energy 

 changes involved, but there is no means of predicting the rate at which it will occur. 

 Practically, therefore, it is not possible to predict with certainty the occurrence of even 

 the simplest reaction, so that it is not surprising that such complicated reactions as 

 those involved in biological systems are still unaccounted for. 



