1905.| Substance in Serum which influences Phagocytosis. 521 
immune serum. On removal of the cocci by the centrifuge the supernatant 
fluid, when tested with fresh cocci, was found to have lost little, or none, of 
its original strength ; whereas the same fluid through which fresh cocci had 
been centrifuged had lost practically all its power. The converse experiment 
gave a quite similar result. In this case, however, care must be taken to 
avoid the cocci being overloaded with immune serum, in which case they are 
capable of giving off some of their substance into the suspending fluid. 
In this case also it is more convenient to use unheated normal serum, which 
enables one to easily estimate the opsonic power by Wright and Douglas’ 
method. These two groups of experiments are strongly in favour of the sub- 
stance in normal serum being identical with the substance in immune serum. 
The relation which the “opsonin” of Wright and Douglas bears to the 
“immune substance,” or “ fixateur” in so far as the latter influences phago- 
cytosis, as shown by Savtschenko among others, must be briefly discussed. 
If the “opsonin” of normal serum were completely destroyed by heating 
to 60° C. for 15 minutes we should be compelled to assume that it was a 
separate and new body, and that the increase in the serum of the property of 
preparing the microbes for phagocytosis, which results from the injection of 
bacilli, was due to an entirely different body, since the substance resulting 
from such bacterial injections is markedly thermostable, even when tested by 
Wright and Douglas’ own method. 
The experiments which are recorded in this paper, however, show that the 
destruction by heating of the “opsonin,” even of normal serum, is only 
fractional, and that its apparently complete disappearance is due to the 
method of observation employed, which demonstrates its presence over a very 
short range. According to the ordinary use of the word in such investiga- 
tions the body is thermostable. 
The fact that this specific substance is present in small amount in normal 
serum is in accord with the numerous observations of the occurrence of 
Immune substance in normal sera. One need only refer to the normal 
antitoxin (eg., of diphtheria), anti-ferments, etc., and to the fact that the 
bacteriolytic and hemolytic actions of normal serum are due to the presence 
in the serum of an immune substance plus a complement, as has been firmly 
established by the work of Pfeiffer, Bordet, Moxter, Ehrlich and Morgenroth, 
and others. In giving the name of “opsonin” to the substance which 
becomes attached to the micro-organisms and prepares them for phagocytosis, 
Douglas and Wright have, therefore, named a property of serum which had 
already been recognised by a number of different workers. | 
Whether free complement may take part in the preparation of the microbe 
is difficult to determine. From the experiments detailed in this paper it is 
