158 IMMUNITY. THEORIES OF IMMUNITY 



mental data. Metchnikoff maintains that it is the cells that absorb 

 the "toxin" that furnish the antitoxin. In other words, the enzymes, 

 as microcytase and macrocytase, exert their action not only upon the 

 more complex molecules of microorganisms, but also upon their simpler 

 toxins, fixing or otherwise altering them until they can finally be de- 

 stroyed. This explanation would lead us to conclude that the nerve- 

 cells which bind the tetanotoxin are capable of furnishing antitoxin, 

 whereas experimental observations are absolutely opposed to this 

 narrower view. Metchnikoff also maintains that antitoxin acts by 

 stimulating the leukocytes to absorb and destroy toxin, whereas Ehrlich 

 has clearly shown that antitoxin, by combining chemically with the 

 toxin, neutralizes it, a process that may be shown in vitro entirely in- 

 dependent of cells. 



From what has been said it will be seen that the two theories are not 

 essentially divergent, and that we are unwarranted in clinging to one 

 view to the absolute exclusion of the other. The question rests largely 

 on which of the body-cells are most active in forming antibodies, and 

 also on a recognition of the role played by phagocytosis in certain in- 

 fections, such as staphylococcus, streptococcus, and pneumococcus in- 

 fections. Ehrlich has attempted an explanation of the method by which 

 body-cells form antibodies, and the manner in which these antibodies 

 overcome their antigens; he has placed both processes upon a chemical 

 basis, involving no one particular group or class of cells. Metchnikoff, 

 on the other hand, has shown the important role played by phagocy- 

 tosis in many infections, and claims that the antibodies in the cir- 

 culating fluids are the products of these phagocytes; he places immunity 

 more largely upon a physical basis. 



The various phenomena of immunity cannot be ascribed either to 

 the activity of the body-cells or to the body fluids alone, to the total 

 exclusion of the other both are intimately concerned in the various 

 phases of immunity. 



It is, moreover, becoming more obvious that too little attention has 

 been paid to the influence of the microorganism in the phenomena of 

 immunity reactions. It is important to recognize that some bacteria 

 are apparently able to immunize themselves against the combative 

 forces of their hosts, as is demonstrated by the manner in which strepto- 

 cocci and pneumococci protect themselves with a capsule and resist 

 phagocytosis. Virulent strains and " resistant races" may be evolved 

 in this manner. This has been demonstrated by Ehrlich with regard 

 to the action of various arsenical compounds on protozoa, work that 



