PHAGOCYTOSIS 333 



That fully virulent living bacteria can be so taken up has been vari- 

 ously proven. The phagocytosis 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 micro- 

 organisms. That the ingested bacteria are often alive after ingestion 

 is proved by the fact that the injection of exudate 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 cyto- 

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

 similar to those mentioned in discussing the digestive processes of 

 amebae. 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 intra- 

 cellular digestion, the bacteria will lose their form, and appear swollen, 

 granular, and vacuolated, and finally will be 110 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 

 animal's 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 

 response 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. 



