OPSONINS. 153 



of the phagocytes, the destructive action of which 

 on some bacteria is well known. Many micro- 

 organisms, indeed, which are not destroyed by 

 serum at all, are readily ingested and killed by 

 leucocytes (staphylococci, streptococci). 



As a consequence of fundamental studies by Phagocytosis 

 Denys and Le Clef, by Leishman, by A. E. Wright 

 and others, it is now possible to study phagocytosis 

 in vitro with a degree of accuracy not to be approx- 

 imated in the living body. It has been learned that 

 leucocytes, as a rule, are not able to take up bac- 

 teria until the latter have been acted on (sensi- 

 tized) by some substance which is contained in the 

 serum. Wright gave the name of "opsonins" to 

 these substances. Some of them are susceptible to 

 heat (55 C.), while others are much more resist- 

 ant. To the latter,, which are greatly increased by 

 infection or artificial immunization, Neufeld has 

 applied the term "bacteriotropins." 



Irishman's method is to mix a suspension of 

 the bacteria with defibrinated blood (containing 

 leucocytes), incubate the mixture for fifteen or 

 more minutes, to make and stain spread prepara- 

 tions on slides, and then count the number of bac- 

 teria which have been ingested by the leucocytes. 

 The average number taken up by fifty or more leu- 

 cocytes constitutes the "phagocytic index." Wright 

 has varied this technic in order to study the opson- 

 ins quantitatively, as will be described later. 



Experiments have shown that all normal serums 

 contain opsonins for a variety of micro-organisms, 

 and all micro-organisms are susceptible to phagocy- 

 tosis by the blood of one or more animals, in case 

 their virulence is not excessive. We have every 

 reason, therefore, to believe that phagocytosis is 



