SUSCEPTIBILITY AND IMMUNITY. 239 
four out of thirty-nine animals experimented upon, and these ani- 
mals succumbed to the anthrax infection in a shorter time than did 
the other five in which there was no such diminution. 
It seems probable that the germicidal property of freshly drawn 
blood serum is not due to its alkalinity, per se, but to the fact that 
the germicidal constituent is only soluble in an alkaline fluid. The 
researches of Vaughn, McClintock, and Novy indicate that this ger- 
micidal constituent is a nuclein. Dr. Vaughn, in his last published 
paper upon “Nucleins and Nuclein Therapy,” says: “Kossel, of 
Berlin, has confirmed our statements concerning the germicidal 
action of the nucleins. Dr. McClintock and I have also demon- 
strated that the germicidal constituent of blood serum is a nuclein. 
This nuclein is undoubtedly furnished by the polynuclear white 
corpuscles.” Denys has (1894) reported the results of experi- 
ments made in his laboratory by Van der Velde, which give sup- 
port to the conclusion reached by Vaughn. In these experiments a 
sterilized culture of staphylococci was injected into the pleural cavity 
of rabbits in order to obtain an exudate. At intervals of two hours 
this exudate was obtained by killing one of the animals in the series 
experimented upon, and at the same time blood from the animal was 
secured. Both the exudate and the blood were placed in a centrifugal 
machine, in order to obtain a serum free from corpuscular elements. 
The germicidal activity of the serum was then tested. The general 
result of the experiments was to show that the longer the interval 
after the injection into the pleural cavity the more potent the ger- 
micidal activity of the exudate became, and that there was no corre- 
sponding increase in the activity of the blood serum obtained from 
the circulation. At the end of ten or twelve hours, the serum from 
the exudate killed all of the staphylococci in a bouillon culture twenty 
times as great in quantity as the germicidal serum used in the ex- 
periment. The absence of any increase in germicidal power in the 
blood serum taken from the general circulation shows that the nota- 
ble increase manifested by the exudate was due to local causes; and 
as a matter of fact it corresponded with an increase in the number of 
leucocytes as found in the pleural exudate. 
Thus it will be seen that the independent researches of Hankin, 
of Buchner, of Vaughn, and of other competent bacteriologists, have 
led them to the same ultimate result so far as the origin of the ger- 
micidal constituent of the blood is concerned, and that the leucocytes 
appear to play an important réle in the protection of the animal body 
from invasion by bacteria (natural immunity). 
It has been shown by several investigators that the number of 
leucocytes increases in certain infectious diseases, and this increase, 
together with an increased alkalinity of the blood, which has here- 
