PILECIPITINS 469 



for several years been generally accepted in place of the older 

 one of MetchnikofE and his pupils, that invading bacteria are 

 dealt with by the leucocytes by a process of phagocytosis. 1 



By an accurate study of haemolysis it was hoped that a 

 sufficient knowledge of the exact nature of the kindred process of 

 bacteriolysis would be forthcoming, whereby a cure for these in- 

 fections due to bacterial invasion could be obtained. Such a hope, 

 however, has not been realised, and very recently a revival of the 

 older theory of phagocytosis has occurred, a revival due in main to 

 the discovery by Wright and Douglas ( 17> l8 ) of certain substances 

 in normal serum which so act on bacteria as to render them 

 susceptible of being taken up by the leucocytes. 



Let us consider one or two of the main experiments which 

 demonstrate this power of normal serum. To measure the phago- 

 cytic power of leucocytes, Wright and Douglas mixed a suspension 

 of washed blood corpuscles, or other fluid containing leucocytes, 

 with an equal bulk of a suspension of bacteria (e.g. B. typhosus, 

 D. pneumonia, B. coli, streptococci. &c.), and placed the mixture 

 in the incubator at 37 C. for about fifteen minutes ; they then made 

 a smear of the suspension and stained it in such a way as to 

 demonstrate the bacteria which had been taken up by leucocytes. 2 

 In this way they were able to count the average number of 

 bacteria taken up by each leucocyte. 3 



If leucocytes, washed free of serum by isotonic saline, be 

 treated, as above, with a suspension of bacteria, the leucocytes 

 do not take up any of the bacteria, but if normal blood serum 

 be present in the mixture then each leucocyte may take up as 

 many as 25 bacteria. Serum evidently, in some way, stimulates 

 phagocytosis. How does it do this? It might be that the 

 serum had stimulated the leucocytes to increased phagocytic 

 power ; or that it had not affected the leucocytes themselves, but 

 had rendered the bacteria more susceptible to being taken up by 

 them. The latter was shown by Wright and Douglas to be 

 actually what occurred, for if the bacteria be first incubated 

 with some normal serum, and then washed free of all trace of 

 adherent serum by saline, they are readily taken up by the 



1 In this older theory it is supposed that the leucocytes, in virtue of their 

 power of phagocytosis, catch hold of the invading bacteria, and by including 

 them in their protoplasm render them innocuous unless the bacteria be of such 

 virulency as to poison the leucocytes, and thus paralyse their phagocytic power. 



2 i.e. by Leishman's method. 



3 It is the polynuclear leucocytes which act as phagocytes. To obtain the 

 average, the bacteria taken up by about twenty-five phagocytes should be counted. 



