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OBSERVATIONS ON THE PROTECTIVE ACTION 
OF NORMAL SERUM IN EXPERIMENTAL INFECTION WITH 
BACILLUS DIPHTHERIAE. 
By T. J. Mackie, M.D., Ch.B., D.P.H. 
(From the Department of Bacteriology, University of Cape Town.) 
In carrying out test inoculations in guinea-pigs with B. diphtheriae 
and diphtheroid bacilli for purposes of identification, where control animals 
were injected with the particular strain plus a certain amount of diphtheria 
antitoxin, it was noted that these animals were protected against lethal doses 
of B. diphtheriae by normal horse serum as well as by the specific immune 
serum. Though this observation did not involve any quantitative com- 
parisons between the effect of the normal and the immune serum, the fact that 
normal horse serum should act protectively in the same way as the serum of 
a specifically immunised animal seemed of considerable importance, and 
suggested the further investigation of the phenomenon. This normal-serum 
effect is also of general interest in relation to " non-specific therapy, " 
where alien protein (e.g. animal serum, bacterial protein, etc.), injected 
parenterally, is found to exert a favourable influence on the course of various 
infections in a non-specific manner. 
With a view, therefore, to analysing the effect, a series of experiments 
were carried out which fully established the original observation, and yielded 
further information of some theoretic and practical interest. 
The protective and curative action of normal horse serum in B. diph- 
theriae infection and intoxication has also been studied by other workers. 
Kolle and Schlossberger concluded that in guinea-pigs infected with 
B. diphtheriae normal horse serum had a limited curative effect, which was 
in no way comparable with that of an antitoxic serum, and in the case of 
animals injected with diphtheria toxin had a certain " delaying " action 
but was not definitely curative. They stated that the effect was " non- 
specific, resistance-increasing, stimulating," but did not offer any further 
explanation of their results. Kraus and Sordelli claimed that normal horse 
serum contains antitoxin to diphtheria toxin, and in virtue of this exerts a 
" preventive " effect against B. diphtheriae and its toxin. Cowie and 
Greenthal showed that 1 c.c. of normal horse serum injected subcutaneously 
