INFECTION, IMMUNITY, AND RECOVERY. 125 
small amount of specific serum, which robs the tissues 
of nothing, produces the same immunity. Again, the 
injection into the body of a sufficient number of patho- 
genic bacteria gives rise to an infection in all cases. 
The theory of Metchnikoff, that the leucocytes or 
wandering cells of the body, after an infection with a 
certain variety of bacteria, become influenced in some 
way, so that they attack especially that form of infection 
again and destroy the bacteria (phagocytosis), can no 
longer be considered as more than a very partial ex- 
planation, and can only be accepted by assuming as 
proven a number of hypotheses. 
The retention theory of Wernich and Chaveau, some- 
what modified, has much to support it. The blood- 
serum of animals recovering from an infection was found 
to have changed chemically to such an extent as to be 
capable of being demonstrated experimentally, and 
these changes were shown to persist for a number 
of weeks or months or even years. Similarly the serum 
of immunized animals retains for a long time its immu- 
nizing substances We are, therefore, compelled to ac- 
cept the fact, that when an infection is passed through 
there are more or less protective chemical substances 
left in the blood, which remain there for a considerable 
time. Kruse believes that these substances have the 
power of neutralizing the bacterial poisons which are 
given off by the bacteria upon their entrance into the 
body, and of thus robbing them of their deleterious 
effects on the alexines; the body fluids in this way 
remaining unsuitable soil for the growth of the bac- 
teria, the alexines being bactericidal. If only a small 
amount of antilysines are present some of the alexines 
are destroyed and the bacteria are not all killed or 
