276 AGEICULTURAL AND INDUSTRIAL BACTERIOLOGY 



Immunity 



Immunity may be defined as resistance to disease. Its 

 converse fs susceptibility. The general fact that individuals 

 vary greatly in their ability to resist disease has of course 

 been known since history began. Many explanations were 

 suggested to explain the reason for an individual becoming 

 immune to a disease after having once been attacked by it, 

 as by smallpox, for example. Some five of these theories 

 are worthy of note. Three of the earlier theories have been 

 quite generally discarded as imperfect or not tenable. Two 

 of the others, however, have played a very large part in the 

 development of modern views. 



Exhaustion Theory of Immunity. — ^When it was recog- 

 nized that bacteria and other microorganisms sometimes 

 cause disease it was suggested that an individual became 

 immune to the disease caused by such an organism due to 

 the using up or abstraction from his body of some of the 

 material essential for the growth of the organism. As long 

 as this deficiency existed bacteria of that type could not 

 grow in the body. It was necessary to discard this concep- 

 tion, however, when it was shown that the blood, for 

 example, of individuals entirely immune to certain diseases 

 contained all of the nutrients necessary for the active 

 growth of their causal organisms. 



Noxious Retention Theory. — When bacteria are culti- 

 vated in the laboratory, it is always noted that they grow 

 most rapidly in the early stages of development. As time 

 progresses the growth becomes slower and slower and finally 

 ceases. It was suggested early that this stopping of growth 

 in culture media was due to the development of substances 

 injurious to the organism. Bacteria growing in milk, for 

 example, produce lactic acid in sufficient quantities event- 

 ually to stop the growth of the organism producing the acid. 

 Reasoning by analogy it was then suggested that bacteria 



