OPSONINS OR BACTERIOTROPIC 

 SUBSTANCES 



Historical. - - The early work of Nuttall and 

 others on the bactericidal action of normal serum, 

 and Pfeiffer's demonstration of the bacteriolysis 

 of cholera and typhoid bacilli by immune sera in the 

 absence of cells, formed the chief basis on which 

 rested the humoral theory, which attributed the 

 protection in such cases to the destructive action 

 of the serum on the microbes. It was found, how- 

 ever, that cases of protection resulting from the 

 use of immune serum occurred where no such 

 bacteriolytic action could be demonstrated; infec- 

 tion with plague or streptococcus may be men- 

 tioned as examples. It is now pretty generally 

 accepted that immunity in these cases is due largely 

 to the phagocytic action of the leucocytes. As far 

 back as 1858 Haeckel had observed that particles 

 of indigo injected into the veins of certain molluscs 

 could shortly afterwards be found in the blood 

 cells of the animal. However, the significance of 

 this and other observations was not appreciated 

 until Metchnikoff * in 1883 called attention to their 

 bearing on infection and immunity. The outcome 



1 Arbeiten des Zoo log. Institutes in Wien, 1883, Vol. v. 



127 



