OPSONIC ACTION 187 



bactericidal action of normal serum. Thus, while we admit 

 that in certain cases a substance behaving like a natural 

 immune-body may be concerned in normal opsonic action, 

 we hold that an opsonic effect on an organism may be 

 due to the direct union of labile opsonin as above ex- 

 plained. 



We have next to consider the question of the opsonic 

 action of an immune serum. Its increased action as com- 

 pared with normal serum is due, as already shown, to the 

 combined action of the specially developed anti-substances 

 acting along with the normal complement-like opsonin, 

 the latter not increasing during the process of immuniza- 

 tion. 



The question is as to the nature of the co-operation of the 

 two substances. Now it is to be noted, in the first place, 

 that the presence of an immune-body of the ordinary type 

 might lead to increased opsonic effect, as it has been shown 

 above that such an immune-body leads to increased absorp- 

 tion of normal complement opsonin ; a greatly increased 

 amount of complement per bacterium is brought into union. 

 And Browning has shown in one case that increased opsonic 

 effect is got when organisms are treated first with immune- 

 body and then with complement, whereas on reversing the 

 order of treatment no such augmented effect is obtained. 

 This, however, is not the whole matter, as the anti- 

 body which we have called ' immune opsonin ' produces 

 the opsonic effect by itself produces, that is, an effect 

 similar to that brought about by normal complement. 

 To this we have no analogy in the case of haemolysis or of 

 bactericidal action. That the increase in the opsonic 

 action of fresh immune serum is in part due to this 

 independent action of immune opsonin there can be no 

 doubt. There remains, however, a further point, namely, 

 whether the molecules in the anti-serum which act as 

 immune opsonins have also the constitution of immune- 



