236 SUSCEPTIBILITY AND IMMUNITY. 
to the highly alkaline reaction of the blood and tissue juices of this 
animal. Behring claims to have obtained experimental proof of the 
truth of this explanation by feeding white rats on an exclusively 
vegetable diet or by adding acid phosphate of lime to their food, by 
which means this excessive alkalinity of the blood is diminished. 
Rats so treated are said to lose their natural immunity, and to die as 
a result of inoculation with virulent cultures of the anthrax bacillus. 
The experiments of Nuttall, Behring, Buchner, and others have 
established the fact that recently drawn blood of various animals 
possesses decided germicidal power, and Buchner has shown that 
this property belongs to the fluid part of the blood and not to its 
cellular elements. It has also been shown that aqueous humor, the 
fluid of ascites, and lymph from the dorsal lymph sac of a frog 
possess the same power. This power to kill bacteria is destroyed by 
heat, and is lost when the blood has been kept for a considerable 
time, but it is not neutralized by freezing. Further, this power to 
destroy bacteria differs greatly for different species, being very de- 
cided in the case of certain pathogenic bacteria, less so for others, 
and absent in the case of certain common saprophytes. Behring 
has also shown that the blood of different animals differs consider- 
ably in this regard, and that the blood of the rat and of the frog, 
which animals have a natural immunity against anthrax, is espe- 
cially fatal to the anthrax bacillus. The experiments made show 
that this germicidal power is very prompt in its action, but that it is 
limited as to the number of bacteria which can be destroyed by a 
given quantity of blood serum. When the number is excessive, de- 
velopment occurs after an interval during which a limited destruc- 
tion has taken place. It would appear that the element in the blood 
to which this germicidal action is due is neutralized in exercising 
this power ; and as, independently of this, blood serum is an excel- 
lent culture medium for bacteria, an abundant development takes 
place when the destruction has been incomplete. 
Buchner (1889) first proved by experiment that the germicidal 
power of the blood of dogs and rabbits does not depend upon the 
presence of the cellular elements, but is present in clear serum which 
has been allowed to separate from the clot in a cool place. Exposure 
for an hour to a temperature of 55° C. destroys the germicidal action 
of serum as well as of blood. 
The researches of Buchner, of Hankin, and others, show that this 
germicidal power of fresh blood serum depends upon the presence of 
proteids, to which the first-named bacteriologist has given the name 
of “alexins.” Hankin, in his paper upon the origin of these “defen- 
sive proteids” in the animal body (1892), arrives at the conclusion 
ae 
