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. 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. 
