IMMUNITY 123 



lieved that as bacteria developed in the tissues they used up 

 some substances necessary to their existence, and that when this 

 substance was exhausted they could no longer grow for lack of 

 proper food. Chauveau took an exactly opposite view and pro- 

 posed his " retention theory." He suggested that as in a test 

 tube bacterial growth may cease because the organisms have pro- 

 duced substances harmful to themselves and not because the food 

 supply is exhausted, so in the body these substances might be 

 formed and retained there and prevent the future development 

 of the organism. Both of these theories are now only of historic 

 interest. It has been proved that the production of immunity is a 

 much more complicated problem, resembling rather a battle be- 

 tween an invading army and defensive forces, during which both 

 parties are extremely active. 



When bacteria have succeeded in overcoming the normal de- 

 fenses of the body and have invaded the tissues the body cells 

 are by no means overcome ; the battle is really only just about to 

 begin. The presence of the invaders stimulates the body cells to 

 defend themselves by actively producing substances termed 

 antibodies, by means of which they endeavor to rid themselves 

 of the invading force or neutralize their products. 



Antigens and Antibodies. Whatever offensive weapon the 

 parasite may produce, the body cells manufacture a substance 

 especially prepared to combat it. If the toxin of a microorganism 

 is its chief means of inflicting injury, such for instance as the 

 soluble toxin of the diphtheria bacillus, the body cells produce 

 an antitoxin to neutralize it ; if it is necessary to destroy the bac- 

 teria themselves then a substance, bacteriolysin, is poured out 

 on the invaders, which dissolves them. In certain infections, and 

 particularly those due to pyogenic cocci, the polynuclear leukocytes 

 and certain other cells engulf the organisms and carry them off 

 bodily. Another antibody, opsonin, appears to aid the leukocytes 

 in their work. 



The term antigen is generally applied to all substances that 

 cause the formation and appearance of antibodies in the body 

 fluids. This power is not confined to bacteria and their toxins, 



