THE THEORIES OF IMMUNITY. 
119 
much more fatal to the young, while in Texas cattle fever the 
old seem to be more susceptible. 
It is said that the blood of naturally immune animals when 
injected into susceptible animals will counteract the effect of 
certain disease-producing germs. For instance, a small quantity 
of the blood of a frog when injected into the circulation of a 
mouse will render that animal immune to anthrax, but has no 
such action in reference to other diseases. It would seem, then, 
that an animal or person may be immune to certain diseases 
and that the blood of such an individual has, to a certain extent, 
the property of producing immunity in other subjects. 
Just what this immunitive substance is remains to be dis¬ 
covered. Many theories have been advanced but they all lack 
positive proof. 
Before discussing these theories in detail it must be remem¬ 
bered that in susceptible animals pathogenic germs, when intro¬ 
duced into the system, multiply with great rapidity, while in 
non-susceptible animals no such multiplication takes place. 
In naturally immune animals, then, there must be some sub¬ 
stance which prevents this development of the germ, or some 
condition of the system unfavorable to its growth. 
In some the body temperature seems to be the unfavorable 
element; since certain bacteria only grow in very narrow limits 
of temperature. The common fowl is generally supposed to be 
immune to anthrax under normal conditions, but Pasteur has 
shown that they may be rendered susceptible by being refriger¬ 
ated after inoculation by immersing their bodies in cold water. 
On the other hand, Koch has pointed out that sparrows, whose 
normal temperature is as high as that of the fowl, contract the 
disease without refrigeration. From the above we are led to 
the conclusion that the high temperature of the fowl is not the 
only factor brought to bear in rendering the fowl immune to 
anthrax. Gibier has shown that the anthrax bacillus will mul¬ 
tiply in both the frog and fish if kept in water at a temperature 
of 35 degrees centigrade ; both being immune under ordinary 
circumstances. The bacillus of anthrax, however, grows under 
