﻿STUDIES IN PLAGUE IMMUNITY. 273 



We learn from a study of these experiments that the anti-infectious 

 substances also become developed very slowly and in very small quantities 

 in animals immunized against pest. While rabbits which have been 

 given a single, small, intravenous inoculation of either living or killed 

 cholera or typhoid bacilli will develop a serum which in high dilutions is 

 protective for guinea pigs against multiple lethal doses of these organisms, 

 on the other hand, rabbits which have been intravenously inoculated 

 with large amounts of killed virulent or with living, attenuated plague 

 cultures, yield sera which, when tested on rats show apparently no pro- 

 tective power whatever against plague infection. Likewise, as large an 

 amount as 5 cubic centimeters of a serum obtained from a rabbit pre- 

 viously inoculated intravenously with 20 milligrams of artificial plague 

 aggressin proved to possess no anti-infectious power when tested on 

 rats. Monkeys which had been immunized against pest by vaccination, 

 or otherwise by inoculation, and which had been shown to be thoroughly 

 immune by the subcutaneous inoculation of multiple lethal doses of 

 the virulent pest strain, furnished sera which also showed no traces of anti- 

 infectious power when tested on rats. Only in the case of one monkey 

 (number 1357) could a slight anti-infectious power be noted and this 

 animal had received repeated, increasing doses of virulent pest bacilli 

 until it resisted the injection of H agar slant cultures of the virulent 

 pest strain. (See Series 57, p. 288.) This series of experiments is par- 

 ticularly important because it illustrates that the negative results in the 

 demonstration of an anti-infectious action obtained with the serum of 

 monkeys less highly immunized against pest, could not be ascribed to the 

 lack of a suitable complement to complete the reaction in the body of the 

 rat. 



The sera of thirty-three human beings who had been vaccinated 

 against pest by the inoculation of attenuated cultures were also tested, bivt 

 in no instance did they show any demonstrable anti-infections value. 

 However, since animals which had proved themselves, thoroughly immune 

 to pest infection also furnished sera which conferred no greater protec- 

 tion, it would not be reasonable to expect that the human sera would reach 

 a higher power; moreover, the serum of a convalescent plague patient 

 collected five days after the symptoms of the disease had subsided, also 

 showed no anti-infectious value. Only in the case of horse's serum, 

 where the animal had finally been inoculated with repeated, large doses 

 of living pest bacilli, could any marked anti-infectious action be demon- 

 strated and indeed with some of these sera it required as much as 1 cubic 

 centimeter to save the rat from fatal pest infection. Therefore, it is 

 unnecessary to emphasize further that the absence of the anti-infectious 

 substances against pest in sufficient quantities to be demonstrated in a 

 serum can not necessarily be regarded as an evidence of the absence of 



