196 Combining Properties of the Opsonin of an Immune Serum. 



merely the agglutinin ? To this question we can give no definite answer. 

 We have tested three anti-sera, viz., 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 development 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 immunisation, 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 inocula- 

 tion 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 complement -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 combining relationships. 



(2) The thermostable opsonin of the anti-serum investigated is a true 

 anti-substance, and possesses the comparatively specific characters of anti- 

 substances in general ; it is left undetermined whether it has the constitution 

 of an agglutinin or of an immune-body, though certain facts point in favour 

 of the former. 



(3) Emulsions of other organisms other than the organism used in immuni- 

 sation (Staphylococcus aureus) do not absorb the immune opsonin ; on the 

 other hand, they absorb large amounts of the normal complement-like opsonin. 



(4) Powerful complement-absorbers — red corpuscles or bacteria treated 

 with immune-body, or serum precipitate — have no effect on the thermostable 

 immune opsonin, whereas they remove almost completely the labile opsonin 

 of the normal and the immune serum alike. 



