1907. | Observations upon Phagocytosis. — 177 
may be producing opsonin and ingesting.’ If the’ phagocytosis increases 
upon allowing further time, it has to be determined whether this is due to. 
further opsonification, or simply to the fact that the cells are allowed a 
longer time in which to work. And even the termination of further phago- 
cytosis, after an extreme time-allowance, might mean equally either a 
termination of further opsonin production, or that the cells had taken up 
all for which they had capacity. 
Our observations bring out that the immune or active cells, as compared 
with normal cells, although they usually do more work in both immune and 
in normal serum, do more in the immune than in the normal. This points 
to some interaction between the patient’s cells and the patient’s serum; it 
indicates that there is something in the patient’s serum which is in excess, or 
which is not present, in normal serum. If we agree to call this “ something ” 
a stimulin, its presence will not, per se, account for the difference. For such 
a substance would equally stimulate the normal cells to do the same amount 
of work as the immune cells in the immune serum—which is not the case. 
In heated immune serum, again, the immune cell does more work than the 
normal: so that, whether the substance left in the serum after heating is 
specifically bacteriotropic and unable to affect the melanin or cytotropic 
(stimulin), it is clear that the immune cells behave differently from the 
~ normal. We are led to hold, therefore, that the immune cell, as a cell, is in 
many cases more active, more irritable, or more sensitive, than the normal, 
as it may be less active than the normal cell, or active to the same degree. 
The immune cell has acquired a heightened activity in the body in response 
to the increased function demanded of it to cope with the infective process ; 
its full reserve power has been called out. The fact that more phagocytosis 
occurs in immune serum with immune cells than with normal cells indicates. 
that, in the living body, from a similar interaction, the patient’s cells are 
likewise doing more work than would normal cells, if we imagined, ey., the 
whole of the patient’s leucocytes suddenly replaced in his own plasma by 
normal cells. 
Question 6.—Is opsonin produced, in vitro, by the patient’s cells in the 
patient’s serum ? 
The only thing that would vitiate the foregoing conclusion would be 
evidence of an additional formation of opsonin, i vitro, during the 
20 minutes in which the capillary tube containing the immune serum, the 
immune cells, and the melanin, or bacilli, was being incubated. 
It is conceivable that the immune serum contains a precursory substance 
—an opsinogen—which is converted into opsonin by a ferment produced by 
the cells, and that the increased phagocytosis of immune cells in immune 
