CHAPTER XXVII 



OPSONINS 



Metschnikoff ascribes both natural and acquired immunity to the 

 activity of the phagocytes. In the former the leukocytes fall on the 

 invading bacteria and devour them. In acquired immunity the phago- 

 cytes are stimulated by the attenuated cultures, or vaccines, and learn 

 how to cope with the invaders. The phagocytes constitute the mobile 

 army of defense. Without previous training they are able to destroy 

 certain bacteria; this constitutes natural immunity. By first bringing 

 them in combat with attenuated cultures, which they satisfactorily dis- 

 pose of, they are so trained that they become able successfully to 

 combat virulent forms ; this explains acquired immunity. Both depend 

 on the efficiency and aggressiveness of the phagocytes. In the main, 

 MetschnikofFs facts have been corroborated, even by those who have 

 desired to controvert them. Phagocytosis is more marked in an 

 immunized animal than in a fresh one of the same species. Is this 

 due to increased efficiency on the part of the phagocytes or to impair- 

 ment of the bacteria ? This is the question now before us. 



The fundamental facts essential to the solution of this problem 

 were discovered by Denys and Leclef in 1895. They showed that 

 phagocytes can be obtained and mixed with bacteria in vitro and that 

 phagocytosis may be observed under conditions controlled by the 

 investigator. They established the following: 1. Phagocytosis pro- 

 ceeds in vitro the same as in vivo. 2. When the phagocytes are col- 

 lected from an animal immunized and from one not immunized and 

 these are washed free from all serum and mixed with the immunizing 

 bacteria suspended in either salt solution or normal serum there is no 

 phagocytic action in either tube. The phagocytes from the immunized 

 animal and those from the non-immunized animal are alike impotent. 

 3. When the two kinds of phagocytes are mixed with the bacteria and 

 suspended in the serum of an immunized animal, phagocytic action 

 begins promptly and is equally marked in the two tubes. 



