

1905.] Substance in Serum which influences Phagocytosis. 509° 
Denys and Leclef (1895) and Denys (1897) showed that the serum of 
rabbits immunised against streptococcus had a bactericida) action on the 
streptococcus, but that the serum of the horse had no such action, though it 
possessed protective properties. The immune substance in the one case 
acts as intermediary body between the cocci and the alexin; in the other 
case between the cocci and the leucocytes. 
Denys (1897) made comparative tests of the phagocytic action of different 
sera in vitro. Measured quantities of streptococci were introduced into tubes 
containing leucocytes, and to certain of these were added immune serum, to 
others normal serum. By plating loopfuls taken from these tubes, and 
counting the colonies at various periods, he was able to demonstrate a 
marked diminution in the tubes containing the immune serum, whereas 
in the tubes containing the norma] serum, an increase was observed. 
He found that the leucocyte of the immunised animal was no more active 
as a phagocyte than the leucocyte of the normal animal. The difference 
in the two cases was entirely due to a property of the serum. 
The conclusion arrived at was that the immunity of the rabbit against the 
streptococcus was due to a modification of the serum, which rendered 
phagocytosis possible. The immunity in this case is a humoral property 
acting by the intervention of the phagocytes. 
Mennes (1897), using the same method of experimentation with the 
pheumococcus, obtained similar results to those obtained with. the 
streptococcus by Denys, and concluded that the immunity in this case 
was due to a modification, not of the leucocyte, but of the serum. The 
serum had not acquired any bacteriolytic property, but had itself undergone a 
change, which resulted in the micro-organisms being taken up and destroyed 
by the leucocytes. 
The results of Denys and also of Mennes are not altogether above the 
criticism made by Metchnikoff, viz., that the occurrence of a certain amount 
of agglutination would appear to give a diminution in the number of 
colonies. 
Douglas and Wright (1903), adopting and modifying the method suggested 
by Leishman, have arrived at a very beautiful technique for the study of 
phagocytosis, and they have published a series of papers on the subject. 
The details of the method are too elaborate for reproduction here, but the 
essential point consists in enumerating the bacteria ingested in a number of 
polymorphonuclear leucocytes, and, by division, obtaining an average, which 
is taken as the measure of the phagocytic power of the blood. They find 
that there is present in the normal blood serum a substance which prepares 
the bacilli, so that they are capable of ingestion by the phagocytes. They call 
