288 SOURCE OF OPSONIN 



significance, and we must regard this substance as a device for 

 attaching more complements and more varieties of complement to 

 an invading bacteria than can easily combine with it direct. In 

 other words, we must regard the cytophile group of the ambo- 

 ceptor as being specific, whilst the complementophile group has 

 the modified specificity which we attribute to the opsonins. The 

 presence of amboceptor will therefore enable the bacterium to be 

 prepared for phagocytosis by the concurrent action of many com- 

 plements which otherwise would only be able to attack it with 

 great difficulty. 



And many facts, notably the liberation of endotoxin taking 

 place when bacteriolysis occurs, would lead us to believe that 

 this preparation for phagocytosis is the true function of ambo- 

 ceptor and complements, and that the appearance of the latter in 

 excess is a comparatively rare phenomenon in disease, and when 

 it occurs in enormous amounts (such as is seen in highly immunized 

 animals) is an artificial phenomenon comparable with the enormous 

 amounts of antitoxin seen in antitoxin-horses. Recovery from an 

 attack of disease caused by B. coli may occur without the appear- 

 ance of any amboceptor to B. coli demonstrable by ordinary tests ; 

 there may, nevertheless, be quite sufficient to act as a thermo- 

 stable opsonin. We are far from denying that bacteriolysis ever 

 occurs under natural conditions, but when there are plenty of 

 leucocytes of sufficient functional activity, it is difficult to avoid 

 the conclusion that they would ingest the bacteria when these 

 were sensitized by complement alone, or complement and a little 

 amboceptor, and before this latter substance had been developed 

 in amount sufficient to cause bacteriolysis. This latter process 

 may perhaps be the last line of defence, to be used only if the 

 leucocytes are injured by the toxins or by the high temperature, 

 or if they are present in insufficient numbers. 



It has been pointed out already that there is some reason to 

 think that, whilst complement and amboceptor can each sensitize 

 for phagocytosis separately, they exert a more potent action when 

 both are present. 



As regards the source of opsonin, little is definitely known. 

 If we regard the thermolable opsonin as identical with comple- 

 ment, we shall regard it as probably derived from the polynuclear 

 leucocytes, and this is corroborated by Levaditi's observations 

 on the aqueous humour. Eyre has also shown that the amount 

 of opsonin (to pneumococci) in the serum in pneumonia may be 



