SUSCEPTIBILITY AND IMMUNITY. 261 
G. and F. Klemperer, in 1891, published an important memoir in 
which they gave an account of their researches relating to the ques- 
tion of immunity, etc., in animals subject to the form of septicaemia 
produced by the Micrococcus pneumonie croupose. They were able 
to produce immunity in susceptible animals by introducing into their 
bodies filtered cultures of this micrococcus, and proved by experiment 
that this immunity had a duration of at least six months. They 
arrived at the conclusion that the immunity induced by injecting fil- 
tered cultures is not directly due to the toxic substances present in 
these cultures, but that they cause the production in the tissues of an 
antitoxin which has the power of neutralizing their pathogenic 
action. The toxic substance present in cultures of the “ diplococcus 
of pneumonia” they call “pneumotoxin”; the substance produced in 
the body of an artificially immune animal, by which this pneumo- 
toxin is destroyed if subsequently introduced, they call “ anti-pneumo- 
toxin.” 
Emmerich, in a communication made at the meeting of the In- 
ternational Congress for Hygiene and Demography, in London, re- 
ported results which correspond with those of G. and F. Klemperer 
so far as the production of immunity is concerned, and also gave an 
account of experiments made by Donissen in which the injection of 
twenty to twenty-five cubic centimetres of blood or expressed tissue 
juices, filtered through porcelain, from an immune rabbit into an 
unprotected rabbit, subsequently to infection with a bouillon culture 
of “diplococcus pneumoniz,” prevented the development of fatal 
septicemia. Even when the injection was made twelve to fifteen 
hours after infection, by inhalation, the animal recovered. 
Emmerich and Mastraum had previously reported similar results 
in experiments made upon mice with the Bacillus erysipelatos suis 
(rothlauf bacillus). White mice are very susceptible to the patho- 
genic action of this bacillus. But mice which, subsequently to in- 
fection, were injected with the expressed and filtered tissue juices of 
an immune rabbit, recovered, while the control animals succumbed. 
According to Emmerich, the result in these experiments was due to 
a destruction of the pathogenic bacilli in the bodies of the infected 
animals ; and the statement is made that at the end of eight hours 
after the injection of the expressed tissue juices all bacilli in the body 
of the infected animal were dead. The same liquid did not, however, 
kill the bacilli when added to cultures external to the body of an 
animal. The inference, therefore, seems justified that the result de- 
pends, not upon a substance present in the expressed juices of an 
immune animal, but upon a substance formed in the body of the 
animal into which these juices are injected. 
We have, however, an example of induced immunity in which 
