IMMUNITY. GENERAL DISCUSSION 125 



microorganisms also results in conferring immunity in much the 

 same manner, perhaps, as does the injection of non-virulent 

 cultures. The injection of bacteria, whether dead or alive, in an 

 effort to increase the immunity of the body, is known as vaccina- 

 tion. This is also denned as the production of an infection that 

 will run a benign course. Bacterial cultures are sometimes 

 allowed to stand until a considerable amount of digestion or 

 autolysis has occurred. When the dead or living bacteria are 

 filtered out by means of porcelain filters, the material remaining 

 in solution or the filtrate may be used for animal injection in an 

 effort to confer immunity. In other cases the bacteria are grown 

 in solutions where they produce certain specific poisons called 

 toxins. These latter are removed by filtration and used to inject 

 animals to induce the development of an active immunity. 



Acquired Passive Immunity. Immunity may be conferred by 

 the injection of serum that contains suitable antibodies. These 

 may be, as will be seen later, in the nature of antitoxins, bac- 

 teriolysins, or opsonins. This method of conferring immunity 

 should not be confused with vaccination. The animal passively 

 immunized takes absolutely no part in the development of its 

 immunity. 



Theories of Immunity. Four principal theories of immunity 

 have been held since the acceptance of the germ theory of disease. 

 The first two proposed are now of historic interest only, but the 

 other two are well founded on fact and are generally accepted 

 at the present time. When first proposed, these latter were sup- 

 posed to be antagonistic, but as subsequently modified, they 

 have been found to supplement each other, some facts being 

 explained by the one and some by the other. These theories 

 are worthy of brief consideration and comparison. 



Theory of Exhaustion. It was noted by early laboratory 

 workers that bacteria, yeasts, and molds would grow rapidly 

 when first planted upon a favorable medium, then the rate of 

 growth would become slower, and finally cease. It was concluded 

 naturally that growth stopped because all of certain of the nutrients 

 needed were exhausted. This theory was applied to the growth of 

 microorganisms in the body, and it was believed that immunity 

 was established when certain requisite food materials were ex- 



