88 STUDIES IN IMMUNITY. 



immunization, which phenomenon serves as an index of the bac- 

 tericidal power which this fluid possesses. If an emulsion of vibrios 

 is mixed with preventive serum, they lose their motility, collect 

 into compact clumps and finally undergo transformation.* This 

 effect by serum is rather markedly specific, as we shall consider in 

 detail later on. 



Serum that has been kept for some time or has been heated to 

 60 degrees remains preventive and may still immunize animals, as 

 Fraenkel and Sobernheimf were the first to note, but loses its bac- 

 tericidal power. Under these conditions it also loses, as might be 

 imagined and as we have pointed out, the property of producing 

 granular transformation of the vibrio. But, as we have particularly 

 emphasized, the bactericidal power may be restored to such serum 

 and it may again become able to transform vibrios. Although 



* It had not been distinctly recognized before our experiments that preventive 

 serum, when added in very small amount to an emulsion of the bacteria against 

 which the serum is active, causes their clumping in little masses that float in the 

 fluid. Observations on the clumping action of the serum of an immunized animal 

 had, however, been made. Charrin and Roger (DeVeloppement des microbes 

 pathogenes dans le serum des animaux vaccines, Societe* de Biologic, 1889, p. 667) 

 may be credited with this observation. These authors noted that "a culture of 

 B. pyocyaneus becomes clear and transparent when placed in the serum of an 

 immunized animal. The organisms are collected in tiny clumps which may be 

 separated by shaking the tube, but which fall again to the bottom when left stand- 

 ing; bacilli placed in normal serum show no such peculiarity. In .the serum of 

 refractory animals, however, the appearance is very distinctive; the organisms 

 are united in chains, etc. Finally, these elements tend to collect together and, 

 instead of floating freely, as do normal bacilli, they pile up in little clumps, which 

 explains the clotted appearance of the culture." 



Later on Metchnikoff (Annales Pasteur, 1891, p. 473 et 474) noted the same 

 phenomenon with the vibrio Metchnikovi; he found that this organism develops 

 on the blood and serum of non-vaccinated guinea-pigs as it does in any ordinary 

 fluid medium. In the serum of immunized animals, on the contrary, the culture 

 shows definite clots that float about in a clear fluid on agitation and then fall to 

 the bottom of the tube. He noted the same fact with the organism of pneumonia, 

 and his observation was confirmed two years later (Annales Pasteur, 1893) by 

 Issaef, who, later still, in collaboration with Ivanhoff, found that the same phe- 

 nomenon occurred with a vibrio discovered by the latter. 



Although Metchnikoff was inclined to regard these phenomena of clumping 

 as of general significance, he made no definite statement to this effect, because the 

 phenomenon did not occur in cultures of the organism of pneumo-enteritis on 

 adding the serum of animals immunized against it (Annales de Tlnstitut Pasteur, 

 1892). 



t Hygienische Rundschau, 1894. 



