194 BACTERIOLOGY FOR NURSES 



of home life and the lack of precautions in preventing the sick 

 from infecting the well. 



Immunity. Although recovery from tuberculosis is of fre- 

 quent occurrence the processes involved are extremely obscure. 

 Many attempts have been made to produce artificial immunity in 

 tuberculosis as in other infectious diseases, but so far all have 

 failed. Tuberculous infection appears to differ in many respects 

 from other infectious processes. Tubercle bacilli may invade 

 the tissues and foci develop without sufficient bodily disturbance 

 to attract attention ; also an infection, so far as clinical symptoms 

 are concerned, may be completely cured, yet the focus may remain 

 and though completely walled off and non-progressive may con- 

 tain virulent tubercle bacilli during the whole of the individual's 

 life. 



Koch discovered that infected animals reacted differently to 

 an injection of living bacilli than did normal animals. When 

 healthy animals are inoculated with virulent organisms tubercles 

 develop near the point of inoculation; the infection is usually 

 carried to the various organs and the animal dies of generalized 

 tuberculosis. A tuberculous animal on the contrary shows an 

 immediate and violent reaction. A marked inflammatory area 

 around the point of injection occurs, followed sometimes by necrosis 

 and sloughing, but with no advance of the infection beyond the 

 point of injection. The reaction well illustrates the phenomenon 

 of hypersusceptibility. Following a first injection of the tubercle 

 bacilli or infection by other means the tissue cells offer no imme- 

 diate resistance and the disease progresses. The presence of the 

 organisms, however, so sensitizes the cells that a second invasion 

 is resisted immediately and vigorously, and protecting substances 

 and phagocytic cells are concentrated upon the point where they 

 are most needed, as is evidenced by the prompt inflammatory 

 reaction. 



Koch, as a result of these observations, concluded that the 

 resistance of tuberculous individuals might be further increased 

 by the injection of disintegrated bacteria and the products of 

 their growth, and with this in view he prepared tuberculin. Un- 



