ROLE OF OPSONINS IN IMMUNITY 193 



except that opsonification in itself apparently does not impair the vital- 

 ity of the bacterium, in so far, at least, its viability is concerned. 



Role of Opsonins in Immunity. Although the exact identity of 

 normal and immune opsonins and their relation to other antibodies is as 

 yet unsettled, the important relation they bear to processes of immunity 

 is generally recognized, especially their ability in aiding resistance to 

 infection by facilitating phagocytosis. They are operative in some in- 

 fections more than in others, and they are especially active in those con- 

 ditions in which phagocytosis is recognized as the chief defensive force, 

 as, for example, in pyogenic infections. In these conditions their pres- 

 ence has been taken as a measure (opsonic index of the resistance of the 

 host) and, largely through the researches of Wright and Douglas, a 

 technic for detecting their presence, kind, and quantity in the body- 

 fluids has been devised, the method and information it yields being of 

 value under certain limitations and in some infections. (See next 

 chapter.) 



If experiments in vitro may be taken as an example of what occurs 

 in vivo, it must be true that leukocytes are capable of consuming an 

 enormous number of bacteria. Experiments with washed leukocytes 

 those removed from the influence of serum show that spontaneous 

 phagocytosis is very slight. Metchnikoff declared these experiments to 

 be untrustworthy for the reason that the various manipulations of wash- 

 ing injures the vitality of the leukocytes. When, however, bacteria are 

 opsonized, that is, are exposed to a serum containing opsonins, and then 

 are thoroughly washed, it is found that the washed leukocytes engulf 

 enormous numbers of bacteria, showing that Metchnikoff 's objection to 

 these experiments is unwarranted. Granting, then, that what we call 

 opsonins are substances that facilitate phagocytosis, and that phago- 

 cytosis is a process of great importance, especially in certain infections, 

 we must conclude that opsonins play a very important role in immunity ; 

 in fact, they constitute the very basis of the phenomenon of phagocytosis 

 in the broader meaning of the term. 

 13 



