108 BACTERIOLOGY. 



bacilli and cholera spirilla, etc. After infection with 

 most parasitic bacteria the body resistance to the growth 

 of the same organism is greatly increased; in other in- 

 fections, however, it is but slightly augmented. 



The protective substances held in solution in the 

 blood-serum are clearly apparent in their effects either 

 in preventing the increase of the bacteria or neutralizing 

 the toxic action of their products; chemically, however, 

 they are but little understood, and although some of 

 them have been shown to be to a large extent specific, 

 that is, they are far more efficient in protecting against 

 the special variety of bacteria which produced the infec- 

 tion than against any other, still we have no knowledge 

 of any chemical difference between them. The addition 

 of 0.5 per cent, of carbolic acid injures these substances 

 but slightly. At ordinary temperatures there is a 

 gradual deterioration in value, so that in from one to 

 six months they may become inert. Twenty hours' 

 exposure to a temperature of 60 C. does not destroy 

 them, but one hour at 70 C. does so almost totally. 

 Different protective substances differ as to the rapidity 

 with which they deteriorate. 



Suitable animals after repeated infections gradually 

 accumulate in their blood considerable amounts of these 

 protective substances, so that very small amounts of 

 serum will inhibit the growth of the bacteria or neu- 

 tralize their products. Thus, 0.1 c.c. of a serum from 

 a horse frequently infected by the pneumococcus will 

 prevent the development in the body of a rabbit of one 

 hundred times the fatal dose of very virulent pneumo- 

 cocci, and a few times a fatal dose of less virulent ones, 

 the actual number as well as the virulence of the bac- 

 teria affecting the protective value of the serum. 



