278 INFECTION AND IMMUNITY 



cytosis is, therefore, not simply a removal of the dead bodies of bacteria 

 previously killed by the body-fluids, but represents an actual attack upon 

 living and fully virulent microorganisms. That the ingested bacteria arc 

 often alive after ingestion is proved by the fact that the injection of exu- 

 date containing, so far as can be determined, only intracellular bacteria, 

 has, in several instances, been found to give rise to infection. 



After the bacteria have remained for some time within the cytoplasm 

 of the leucocyte, vacuoles may be seen to form about them, similar to 

 those mentioned in discussing the digestive processes of amebse. If the 

 preparations are, at this stage or later, stained with a one-per-cent 

 solution of neutral red, it will be found that the bacteria, colorless under 

 normal conditions, will be stained pink, an evidence of their beginning 

 disintegration. At a later stage in the process of intracellular digestion, 

 the bacteria will lose their form, and appear swollen, granular, and 

 vacuolated, and finally will be no longer distinguishable. If, on the 

 other hand, the ingestion of bacteria brings about the death of a leucocyte, 

 the neutral red will not stain the bacteria, the digestive vacuoles will not 

 form, and the leucocyte itself will disintegrate. 



It must not be forgotten, however, that not all microorganisms are 

 equally susceptible to phagocytosis. Some may resist ingestion more 

 energetically than others by agencies not fully understood. Others 

 again, like the tubercle bacillus and the anthrax bacillus for instance, 

 may, after ingestion, oppose great difficulties to intracellular digestion. 



To a certain extent, moreover, the variety of the bacterium deter- 

 mines the variety of phagocyte attracted to the point of invasion. In 

 the cases of most of the bacteria of acute diseases, the microphages or 

 polymorphonuclear leucocytes are the ones upon which the brunt of 

 the battle devolves. Other invaders, like the Bacillus tuberculosis, 

 blastomyces, and others, find themselves opposed chiefly by the macro- 

 phages. Cells of animal origin, such as the dead or injured cells of the 

 animals' own body or the cells of other animals artificially introduced, 

 are ingested by macrophages. This is true also of many parasites of 

 animal nature. 



It is clear, thus, that the process of phagocytosis is a universal re- 

 sponse on the part of the body to the invasion of foreign particles of 

 dead material, of alien cells, and of living microorganisms. It remains 

 to be shown upon what basis this process may be regarded as an essential 

 feature in protecting the body against infection. 



The numerous researches of. Metchnikoff have brought out the 

 important fact that phagocytosis is regularly more active in cases in 



