168 INFECTION AND IMMUNITY. 



streptococcus, staphylococcus, pneumococcus, and 

 several others. 



je fn c Aetive It was stated in the section on natural immun- 

 fry fa^ ^ leucocytes, acting as phagocytes and 

 as resorptive cells, seem to be responsible, at least 

 in part, for natural resistance to an infection, and 

 there is now no lack of evidence to show that they 

 are of great importance for acquired immunity, at 

 least in many instances. Particularly, Metchni- 

 koff and his followers have provided us with many 

 observations which go to prove this point. 



These investigators showed that in acquired 

 immunity the phagocytes have a much greater 

 capacity for ingesting and killing bacteria and for 

 absorbing and destroying toxins than when the 

 animal is in a state of greater susceptibility. It is 

 also concluded that the serum in active immunity 

 owes its new or more powerful antibacterial, anti- 

 toxic and other properties to the leucocytes, which 

 under the influence of the infection have produced 

 these substances in excess and excreted them into 

 the plasma. 



The views of Metchnikoff regarding the impor- 

 tance of phagocytosis have been greatly strength- 

 ened in recent years as a consequence of quantita- 

 tive studies of phagocytosis in vitro. As already 

 stated, phagocytosis of bacteria depends on their 

 first being "sensitized" by the opsonins which are 

 present in the serum. In 1895, Denys and Le 

 Clef demonstrated that the serum of animals 

 which had been immunized with streptococci 

 induced a much greater phagocytosis and destruc- 

 tion of these organisms than normal serum, deter- 

 mining their results by means of plate cultures 

 and microscopic studies. More recently, by means 



