THE DEFENSES OF. THE BODY 277 



washed leukocytes be brought together the bacteria are not 

 taken up by the cells; if on the other hand a drop of normal 

 serum be added, phagocytosis begins. Or, if bacteria be 

 immersed in normal serum and then carefully cleaned of all 

 adherent serum by washing they will readily be taken up 

 by leukocytes, even those also freed of all serum by careful 

 washing. In short the action of the serum on the bacteria, 

 through its " opsonin," has been to make them ingestible or 

 digestible for the leukocytes. 



This opsonizing property of the blood varies. Under 

 conditions depressing general health it may be diminished; 

 while in the course of infective diseases it is sometimes 

 lessened, sometimes increased. It may be increased by 

 immunization. 



The nature of opsonin (or opsonins) is not known. It has 

 been suggested that they are allied to the enzymes. They 

 are destroyed by heat. They may be absorbed entirely 

 from the blood by bacteria with which they combine. They 

 are unstable, becoming gradually inert after withdrawal 

 from the body. 



In consequence of these later investigations the phago- 

 cytes are again to the fore as one at least of the important 

 defenses of the body and certainly, in so far as the destruction 

 of invading bacteria is concerned, many have come to look 

 upon them as, after all, just what Metchnikoff originally 

 regarded them, the true scavengers of the body. 



Though the destruction of bacteria by the fluids of the 

 body had been demonstrated; though their inclusion and 

 digestion by phagocytes could readily be observed; though 

 an antidote for certain of their poisons could be demon- 

 strated in the blood of immunized animals, there was still 



