522 Mr. G. Dean. On the Nature of the [July 8, 
certain that it is not a necessary participant in this action. At the same 
time it is not improbable that the immune body when aided by complement 
may act more powerfully, and that the sudden fall in the “ opsonic” power of 
both normal and immune serum on heating is due to the destruction of the 
complement. I may revert to this subject on another occasion. 
Metchnikoff’s statement in regard to natural immunity, that the leucocytes 
undertake the struggle against the microbes and free the organisms from 
them without the need of previous help on the part of the humors, is 
apparently largely based on Bordet’s and Gengou’s results obtained by their 
method of testing for the presence of “fixateur.’ The results obtained by 
the use of that method are not above criticism. That the amount of fixateur 
present in normal serum compared with immune serum is small is true, but, 
as suggested by Savtschenko, the amount necessary to prepare the organism 
for phagocytosis may be small compared with that necessary for bacteriolysis. 
When microbes, ¢.g., streptococci, injected into the peritoneal cavity, come in 
contact with the phagocytes, at first they are englobed by these, but soon 
some are observed to be free and to multiply rapidly, apparently having the 
power of repelling leucocytes. Bordet and Savtschenko interpret this to 
mean that the cocci have acquired during this brief period a new property 
which gives rise to a negative chemiotaxis of the leucocytes. 
In the light of my experiments another view may be taken, viz., that the 
first organisms injected are phagocyted, because they have been sensitised 
by the immune substance present in the normal serum. This being small in 
amount is soon exhausted, and the few organisms which may have escaped 
its action are able to multiply, and either are indifferent to the leucocytes or 
exercise their repelling influence on them in the absence of the naturally 
present immune substance. In such a case the indifference displayed by the 
phagocytes to the cocci, or it may be the repulsive force of the cocci, is not a 
newly-acquired property, but is inherent in the cocci, and is only overcome 
by the presence of the immune substance which acts as intermediary between 
the micro-organism and the leucocytes. 
Conclusions. 
1. That, as has been shown by a number of workers, ¢.g., Denys, Metchni- 
koff, Savtschenko, Levaditi and others, there is produced in the blood serum 
of animals actively immunised by bacterial injections a specific immune 
substance which has among its properties that of preparing the microbe for 
phagocytosis. | 
2. That this immune substance is thermostable, resisting a temperature o 
60° C. for several hours. 
