6 QiJATERCENtENARY STUDIES IN PATHOLOGY 



and which, though capable of acting to a certain extent independently, 

 may have its action enormously reinforced by co-operation with a 

 thermolabile complement. In a recent paper by R. D. Keith (^s), 

 the view is also put forward that the immune opsonin is partially 

 thermostable, but it is not identical with the haemolytic amboceptor, 

 as Dean believed. To explain the residual opsonic action that takes 

 place when immune serum has been heated, Keith suggests that, in 

 addition to its cytophilic and complementophilic groups, the hypothetical 

 opsonic amboceptor may possess a special thermostable opsoniphoric 

 group, enabling a certain amount of action to proceed in the absence 

 of complement. Into the problems involved in this highly theoretical 

 subject, it would be out of place to enter here. 



We take as our main premiss, the standpoint, that the opsonin is a 

 simple thermolabile body residing in the blood serum, whose action 

 m vitro is exerted solely on the micro-organisms concerned, and is 

 entirely independent of the leucocyte. 



The ultimate origin of the opsonic substance, like that of the 

 bactericidal and other protective bodies, is difficult to determine with 

 precision, and it was felt that light might be thrown on the subject, 

 not so much by Buchner's method of analysing separately the cellular 

 and fluid portions of the plasma as to their opsonic content (though this 

 also was attempted), as by estimating what influence, if any, was exerted 

 by the presence of hyperleucocytosis on the opsonic content of the 

 plasma, or, more correctly, of the cell-free serum after coagulation. 



If the opsonic substance is ultimately derived from leucocytic 

 degenerative products, one would expect it to be profoundly influenced 

 by conditions in which the white cells are in great excess in the blood 

 stream. 



The injection of many substances differing widely in chemical 

 constitution has long been known to be followed by a period of hyper- 

 leucocytosis, during which the natural resistance to infection is increased. 

 Indeed, it is probable that in many of the so-called cases of cross- 

 immunisation or vaccination with sterile emulsions of bacteria, whose 

 origin is entirely different from those against which they exert a 

 protective action, the results obtained are attributable solely to the 

 leucocytic variations produced by them. 



It will be convenient here to summarise briefly the protective effects 



(326) 



