188 ANTI-BACTERIAL PROPERTIES OF SERUM 



bodies. If this were so, then the increased opsonic action 

 of a fresh immune serum might be due to the following 

 factors : 



(a) Immune-body (amboceptor) leading to increased com- 

 bination of complement opsonin ; 



(b) Immune opsonin producing opsonic effect by itself ; 



(c) The same molecules in the anti-serum acting both as 

 immune-bodies and as immune opsonins. 



We consider it proved by what has been stated above 

 that both of the two first possibilities hold good. With 

 regard to the third there is still doubt. In a serum 

 where there is a mixture of anti-substances it is a very 

 difficult matter to demonstrate that immune opsonins act 

 at the same time as immune-bodies. If this were so, then 

 we would have a new type of anti-substance ; one which has a 

 zymotoxic group and one which at the same time leads to 

 the absorption of complement. Theoretically there is 

 nothing against such a supposition. In fact it is suggested 

 by the deviation of complement brought about by a pre- 

 cipitin and the homologous serum. We are, however, not 

 justified in saying that all the molecules of immune opsonin 

 have the constitution of immune-body. The fact above 

 referred to, namely: that an immune serum may have 

 marked opsonic action and contain comparatively little 

 immune-body, speaks against such a supposition. It is 

 quite likely that the immune opsonins, in part at least, 

 have the constitution of agglutinins, possessing a zymotoxic 

 group, but not leading to the union of complement. In 

 any case they are bodies of great importance in immunity 

 as they can act in the absence of complement. 



The term ' immune opsonin,' as used above, has practi- 

 cally the same significance as the ' bacterio-tropin ' of 

 Neufeld ; that is, it denotes a specific thermostable anti- 

 substance which, by itself, promotes phagocytosis. This 



