316 INFECTION AND RESISTANCE 



immune serum. This is the more surprising since, in the case of all 

 other antibodies (lysins, agglutinins, etc.), it has been shown that in 

 structure and mode of action the antibodies of immune serum are 

 in every way qualitatively similar to the corresponding ones of nor- 

 mal serum, 18 , 19 representing merely a specific quantitative increase 

 of substances originally present in small amount. 



This difference between the normal and immune opsonic sub- 

 stances has added much difficulty to the investigation of the nature of 

 these bodies, and we may approach the problem with greater clearness 

 by considering them separately, at first, attempting to define the 

 relations between them after we have set down the facts ascertained 

 in connection with each. 



In their earliest investigations upon the normal opsonins Wright 

 and Douglas 20 regarded them as new antibodies, separate and dis- 

 tinct from those already known. There is no convincing proof of 

 this, and a number of other interpretations of the observed phe- 

 nomena are possible. Indeed, the burden of proof is rather upon 

 those who would establish the existence of a new antibody, for before 

 this can be done it must be shown that the new function is not merely 

 another property of the serum constituents already known. For, as 

 Gruber has justly said, "One of the most important attributes of the 

 natural scientist is economy of hypotheses.'' And in the case of the 

 normal opsonins there are many good reasons for regarding them as 

 possibly identical with known serum constituents. The two possi- 

 bilities suggested have been (1) Are the opsonic substances identical 

 with the alexin or complement ? or ( 2 ) Do they represent the com- 

 bined action of the normal sensitizer of the serum activated by the 

 alexin ? 



The similarity of normal opsonin with alexin or complement has 

 been brought out especially by Muir and Martin, 21 by Baecher, 22 

 and by Levaditi and Inmann. 23 The fact that both are thermolabile 

 has been mentioned above. 



In addition to this, as Muir and Martin 24 have shown, all antigen- 

 antibody complexes which absorb alexin out of serum at the same 

 time remove the normal opsonin. Thus sensitized red corpuscles, 

 sensitized bacteria, and specific precipitates added to normal serum 

 take out its opsonic substances. From this fact they also concluded 

 that the normal opsonins like alexin were non-specific. For just as 



18 Dean. Proc. Eoyal Soc., 76, 1905. 



19 Neuf eld and Hiine. Arb. a. d. kais. Gesundh. Amt., Vol. 25, 1907. 



20 "\Vright and Douglas. Loc. cit. 



21 Muir and Martin. Br. Med. Jour., Vol. 2, 1906 ; Proc. Eoyal Soc. B., 

 Vol. 79, 1907. 



22 Baecher. Zeitschr. f. Hyg., Vol. 56, 1907. 



23 Levaditi and Inmann. C. R. de la Soc. Biol., 1907, pp. 683, 725, 

 817, 869. 



24 Muir and Martin. Loc. cit. 



