PHAGOCYTOSIS 31 



case of bacteria, it has been shown that the virulent 

 living organisms can be taken up by the polymorpho- 

 nuclear leucocytes. 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 the living and fully virulent 

 organisms. Some organisms, however, show much 

 greater resistance to phagocytosis than others, while 

 some bacteria, as the tubercle bacillus, after ingestion 

 by the leucocytes, oppose great difficulties to intra- 

 cellular digestion. Cells of animal origin, or the 

 dead cells of the animal's own body, are ingested by 

 the large mononuclear leucocytes. Phagocytosis is 

 much more active in animals which have a high 

 degree of immunity to certain micro-organisms than 

 in those that are more susceptible. 



Opsonins. — Wright, by a study of the relation of the 

 blood serum to phagocytosis in a series of experiments, 

 proved conclusively that the serum contained a sub- 

 stance which acts directly upon the bacteria and not 

 upon the leucocytes. This serum component is bound 

 by the bacteria and renders them subject to phago- 

 cytosis. Because of their action in preparing the 

 bacteria for ingestion by the leucocytes he named those 

 bodies *' opsonins." The importance of these opsonic 

 substances in immunity was show^n by Wright in a 

 series of experiments, in which he determined that in 

 patients ill wdth staphylococcus or tubercle infections 

 the phagocytic powers were relatively diminished 

 towards those micro-organisms, but could be specific- 



