NATURAL RESISTANCE TO DISEASE—FLEXNER. Ton 
Moreover, it is now quite certain that of the two principles the 
intermediary body alone is a fixed, native element of the blood 
plasma, and the complement is subject to considerable fluctuations 
in quantity. The origin of the intermediary body has not been de- 
termined, while it is quite established that the complement is yielded 
by the white corpuscles, or leucocytes, of the blood. This matter of 
the origin of the complement is very important because the protective 
value of the blood fluid is determined by the quantity of comple- 
ment available at any one time and not so much by the more constant 
intermediary body which is usually in excess of the complement. 
The complement would appear to arise from the leucocytes partly as 
a secretion; but the quantity derived in this way would not appear 
to be considerable. It also arises from leucocytes which are brought 
by any cause to degeneration and disintegration, and this would 
seem to be a richer source than the other. Leucocytes are constantly 
being worn out by physiological use and as constantly yielding up 
their complement to the blood as they go to pieces. It would appear, 
then, that the very essential complement which exists in the circulat- 
ing blood and passes from the blood into the lymph and serous 
‘avities, will be more or less determined in quantity by the number 
of blood leucocytes and the conditions to which they are exposed, 
and as they are brought to slower or faster degeneration; and it is 
extremely probable that the secretion of complement is influenced 
also by the nature of the stimuli to which even the living leucocytes are 
exposed. It has been shown beyond peradventure that the blood 
plasma contains less complement than blood serum, as would now be 
expected since the origin of complement from degenerating leucocytes 
has been abundantly shown, and because in the clotting of the blood 
the leucocytes are so greatly disintegrated. But I do not think that 
even the most ardent adversaries of the view that the fluids of the 
interior of the body do not exert direct bactericidal effects have been 
able to show that the plasma contains no complement. The comple- 
ment is such a labile body that doubtless it is constantly used up 
physiologically and must therefore as constantly be renewed, and it 
is highly probable that the balance between production and destruc- 
tion may not always be maintained, whence a considerable fluctuation 
may occur even in health. Whether the fluctuations ever synchronize 
with intending infections in such a manner as to promote them is 
not really known, but is not impossible. 
It is, however, patent that the naturally operative defensive mech- 
anisms against bacterial invasion must contain other factors than 
these humoral ones. We are all now prepared to admit that in the 
phagocytes, or the devouring white corpuscles of the blood, the body 
possesses another defensive system of high efficiency. The motile 
nature of these cells and their presence in the circulating blood ac- 
45745°—sm 190947 
