190 ANTI-BACTERIAL PROPERTIES OF SERUM 



depends both upon the receptors to which the immune-body 

 is attached and also upon the complement. 



A further question arises with regard to cases where the 

 labile complement opsonin is concerned, namely, as to 

 whether or not the substances producing opsonic effect are 

 the same as those which bring about bactericidal action or 

 bacteriolysis. The statement is not infrequently made that, 

 because a serum may produce opsonic effect and no bacteri- 

 cidal action, therefore the substances concerned in these two 

 actions are different. This line of reasoning is quite falla- 

 cious, as no conclusion can be drawn when the two effects are 

 tested on two different organisms. The same substance 

 may quite well produce the bactericidal action on one 

 organism and no bactericidal action on another organism, 

 and yet be capable of producing the change in the latter 

 necessary for opsonic effect. The process of opsonization 

 necessarily means a much less disruptive effect than the 

 change leading to death of the organism ; it may mean 

 nothing more than this, or it may be different in kind. As 

 a matter of fact, organisms may take up bactericidal com- 

 plement, and when treated with the homologous immune- 

 body may take it up in large amount, without any bacteri- 

 cidal effect following, simply because the organism is not 

 sufficiently susceptible to the toxic action of the combined 

 complement. The identity or non-identity of the sub- 

 stances concerned in bactericidal and in opsonic action when 

 complement is concerned, must be left an open question for 

 the present. 



Our general conclusions are that in the case of normal 

 sera the opsonic effect is generally due to the labile non- 

 specific complement ; it may act with or without a natural 

 immune-body. (A close parallelism as regards mechanism 

 may be drawn between this and normal bactericidal action, 

 and the same principles will probably be found to obtain 

 (p. 203).) In the case of immune sera the opsonic effect 



