RESISTANCE TO INFECTIOUS DISEASE 9 



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 determined, 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 complement 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 circulating blood and passes from the blood into the lymph 

 and serous cavities, 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 leuco- 

 cytes 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 complement 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 destruction 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 mechan- 

 isms against bacterial invasion must contain other factors than these 

 humoral ones. "We are all now prepared to admit that in the phago- 

 cytes, 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 accord them a high 

 degree of mobility, so that they can be quickly dispatched to any part of 

 the body threatened by invaders, and are hardly behind the fluids of 



