CHAPTER LVIII 

 THE MECHANISM OF AGGLUTINATION 



JOHN H. NORTHROP 



Rockefeller Institute for Medical Research 



Princeton, N.J. 



The phenomenon of agglutination of suspensions of bacteria in the presence of 

 homologous immune serum was thoroughly described in 1896 by Gruber and Durham. 

 The reaction was then studied by a number of workers and a series of suggestions 

 made as to its mechanism. It was first clearly recognized by Bordet' that the phenom- 

 enon was a special case of the flocculation of a colloidal suspension. Subsequent work 

 has tended only to confirm this conclusion, and it is now generally accepted. In the 

 meantime, much progress has been made in the theory of the flocculation of suspen- 

 sions. In some respects the work with other suspensions is simpler, and a complete 

 picture of the reaction can be obtained only by considering the flocculation of suspen- 

 sions in general. The literature on the subject is very extensive, and no attempt has 

 been made to refer to individual publications other than those which appear to lead 

 directly to the present theory. More general reviews of the subject may be found in 

 Freundlich,^ Burton,^ Bancroft,'' and Taylor^ and recent reviews dealing more specifi- 

 cally with bacterial agglutination in Zinsser'' and Wells.'' 



A suspension undergoing the process of flocculation presents a definite series of 

 changes. At first the individual particles cannot be seen except with the microscope. 

 Their presence is shown, however, by a Tyndall cone when light is passed through the 

 suspension. Larger particles then make their appearance and may usually be seen as 

 discrete clumps. These clumps consist of a number of small particles adhering firmly 

 together but still retaining their individual form. More or less rapid settling of these 

 larger particles now occurs, and in the course of time the solid matter forms a precipi- 

 tate on the bottom of the vessel, leaving a clear liquid above. The suspension is now 

 flocculated or "agglutinated." The appearance under the microscope is similar, ex- 

 cept that, in addition, it can be seen that the small particles are in rapid irregular 

 movement — the Brownian movement — whereas the large clumps are nearly station 

 ary. It can also be usually seen that the particles do not actually coalesce but merely 

 approach one another closely. (In the case of the "breaking" of an emulsion there is 

 actual coalescence; this is a distmct phenomenon and wiU not be considered here.) 



' Bordet, J.: Ann. de I'lnst. Pasteur, 13, 225. 1899. 



^ Freundlich, H.: Ka pillar chemie (2d ed.). Leipzig, 1922. 



3 Burton, E. F.: The Physical Properties of Colloidal Solutions (2d ed.). London, New York, 

 Bombay, Calcutta, and Madras, 1921. 



■» Bancroft, W. D.: Applied Colloid Chemistry. New York and London, 19 21, 



5 Taylor, W. W.: Chemistry of Colloids (3d ed.). New York, 1915. 



'Zinsser, H.: Infection and Resistance. New York: Macmillan Co., 1925. 



'Wells, H. G.: Chemical Aspects of Immunity, "American Chemical Society Monographs." New 

 York, 1925. 



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