528 STUDIES IN IMMUNITY. 



does not serve to differentiate them as clearly as agglutination 

 does. This fact may be compared with the observations of 

 various authors in accordance with which the reaction of fixation 

 does not show as strict a specificity as the reaction of agglutination. 



In so far as the production of antigens which are sensitive to 

 agglutinating antibodies is concerned, we may conclude that the 

 culture medium may be of very great importance. It may still 

 further be shown that these two strains of organisms may be trans- 

 formed into one another very rapidly by changing the culture 

 medium. Thus, following two successive generations on blood 

 medium, the organism that has been previously cultivated on agar 

 recovers the faculty of producing the antigen which is character- 

 istic of blood cultures and consequently becomes agglutinable by 

 the serum of a horse that has been immunized against such cultures. 



I cannot go further into these researches, the details of which 

 would exceed the limits of this short summary, but the little that I 

 have said suffices to justify experimentally the opinion which is also 

 expressed in the work of Grassberger and Schattenfroh, that cul- 

 tures of bacteria developed on culture media which differ too much 

 from the body fluids (for example, grown on bouillon or agar steri- 

 lized in the autoclave), are not suitable, perhaps, to be employed in 

 immunizing animals for the production of therapeutic sera. It is, 

 perhaps, possible that bacteria grown on these two artificial culture 

 media fail to form all the antigens which they produce in the animal 

 body during infection and which would be affected by suitable 

 antibodies. 



*** 



I may conclude this brief resume* at this point. Although I have 

 discussed in some detail the properties of sera, I have not taken up 

 at all the essential topic of phagocytosis. The importance of this 

 phenomenon in the defence of the animal body, which was so 

 much combated fifteen years ago, has no need, at the present day, 

 of emphasis. We have long since passed the time when the exact 

 observations and the decisive experiments of my former master 

 and present friend, Elie Metchnikoff, met with warm but often 

 superficial opposition from those scientists who were too exclu- 

 sively preoccupied by the antibacterial properties of the body fluids. 

 Many facts which have long been known, but the significance of 



