PHAGOCYTOSIS 283 



Let us suppose that the injected bacteria have been small in 

 quantity and moderate in virulence. In such a case a rapid phago- 

 cytosis gradually rids the fluid of micro-organisms and within 24 

 hours after injection few, if any, free bacteria are visible. 



A little exudate taken at this time shows large numbers of micro- 

 phages varyingly crowded with well-preserved and disintegrating 

 bacteria. Some of the phagocytes, having literally taken up more 

 than they can digest, are vacuolated and disintegrating, but, in gen- 

 eral, the victory lies with the cells. A little later large mononuclear 

 elements appear, and here and there will be seen to take up dead 

 leukocytes together with ingested cocci. In this way gradually a 

 cleaning out of the peritoneum takes place, the animal recovers, and 

 the peritoneum returns to normal. 



Let us suppose, on the other hand, that the bacteria injected are 

 in larger doses and of greater virulence. In such a case, after a 

 period of active phagocytosis, there may be a gradual increase of 

 bacteria over leukocytes. The phagocytic cells are found to be under- 

 going degeneration in larger numbers, the free bacteria increase, 

 and the impending death of the animal can often be foretold by the 

 appearance of the exudate. Finally, the peritoneal fluid may con- 

 sist chiefly of free and rapidly multiplying bacteria with a practical 

 absence of phagocytic cells. 



In all of the processes so far as described the burden of the 

 defence has fallen upon the microphages or polynuclear leukocytes, 

 while the macrophages endothelial and connective tissue cells 

 have taken a purely secondary part in the reaction, forming, to some 

 extent, a second line of defence, or, more probably, taking part only 

 in the final removal of degenerated and disintegrating combatants 

 and tissue detritus. In order to obtain a complete conception of 

 phagocytosis in its entire significance it will be necessary to consider 

 a further example, namely, the process which takes place within tis- 

 sues in the course of the efforts of macrophages to remove bacteria 

 and other substances which, either because of their insolubility or for 

 other unknown reasons, are refractory to the attacks of the mi- 

 crophages. Since we are interested in this subject chiefly from the 

 point of view of the defence against bacteria, we may illustrate this 

 process best by the description of the reaction which takes place when 

 tubercle bacilli become localized anywhere within the animal body. 



When tubercle bacilli are injected into the peritoneum they are 

 actively taken up by the polynuclear leukocytes just as are other 

 bacteria and many entirely inactive solid particles. A similar inges- 

 tion by microphages may take place in the folds of the intestinal 

 mucosa if tubercle bacilli are fed to guinea pigs. However, this 

 preliminary phagocytosis is probably of but secondary significance 

 in the combat of the body against tuberculosis, since it has still to 

 be shown that polynuclear leukocytes are capable of digesting and 



