OPSONIC ACTION 189 



writer, in a recent review 1 of the subject, restricts the term 

 ' opsonin ' to cases where the thermolabile complement 

 opsonin is concerned, and considers such opsonic action 

 always to proceed according to the scheme of immune-body 

 (natural or artificial) -f- complement. It is quite evident 

 that the term ' opsonin ' as signifying a body which promotes 

 phagocytosis cannot be applied to any one substance ; it 

 can only be used as signifying a property of a substance, not 

 its constitution. We, however, prefer the term ' immune 

 opsonin ' to ' bacterio-tropin,' as it is sufficiently definite 

 and the term ' opsonin ' is so convenient and now so widely 

 understood. Neuf eld discusses the relation of the bacterio- 

 tropins (immune opsonins) to immune-bodies, and concludes 

 that they are distinct substances. He, however, appears 

 to rely too much on the definition of immune-body. If 

 this definition includes two properties as necessary, namely, 

 (a) that it leads to the union of complement ; and (6) that 

 it produces no recognizable effect by itself ; then manifestly 

 bacterio-tropins are not immune-bodies, as they produce 

 such an effect. (We agree with him in holding that the 

 opsonic action of a bacterio-tropin is not due to complement 

 supplied by leucocytes.) But the important question still 

 remains, namely, whether the same molecule which pro- 

 duces an opsonic effect, by means of its zymotoxic group, 

 has also the property of an immune-body of leading to the 

 union of complement. With regard to this, all that we can 

 say is that, though it is quite probable, we do not consider 

 that it has yet been satisfactorily demonstrated. Proof is 

 equally wanting that all the molecules of immune opsonin 

 also have the property of immune-body referred to. The 

 complexity of the question becomes still more evident when 

 we consider that immune-bodies do not always lead to the 

 fixation of complement ; this, as has been shown above, 



1 Neufeld, * Kolle und Wassermann's Handbuch der pathogenen Milcro- 

 organismen,' Erganzungsband II, Heft ii, p. 303. 



