184 ANTI-BACTERIAL PROPERTIES OF SERUM 



three anti-sera, viz. to the Staphylococcus aureus, the B. coli, 

 and the Vibrio Metchnikovi, and have found that they all 

 possess agglutinating properties, while the anti-coli serum 

 alone has no opsonic action. It is quite possible that the 

 same substances may act at the same time as agglutinin and 

 as opsonin, and that one of these properties may be wanting 

 in any given case ; but at present we have no facts to justify 

 any expression of opinion. 



So far as phagocytosis is concerned, the all-important 

 factor in active immunity would appear to be the develop- 

 ment of an immune opsonin with comparatively specific 

 characters. In testing the opsonic effect of an unheated 

 immune serum the result will represent the sum of the 

 actions of the complement-like opsonin and the immune 

 opsonin. As complements do not appear to increase in 

 amount during immunization, a rise of opsonic index will 

 probably depend upon the development of immune opsonin, 

 and will thus have a certain specific character. Thus 

 Bulloch and Western found that inoculation of a patient 

 with tubercle vaccine produced an increase of tubercle 

 opsonin but not of Staphylococcus opsonin, and conversely 

 inoculation with Staphylococcus vaccine caused an increase 

 of the Staphylococcus opsonin but not of the tubercle 

 opsonin. This appears to be in harmony with the results 

 which we have obtained. On the other hand, a fall in the 

 opsonic index might be due to diminution of the comple- 

 ment-like opsonin, and thus be without specific significance. 

 Further observations will be made on this point. 



GENERAL CONCLUSIONS 



1. The thermolabile opsonin of a normal serum and the 

 thermostable opsonin of an immune serum are two distinct 

 classes of substances. In addition to differing markedly 

 as regards their resistance to heat, they differ in their com- 

 bining relationships. 



