SPECIFIC BACTERIAL PRODUCTS 451 



such a way as to enhance their ingestion by the leucocytes exist 

 in the blood serum of immunized rabbits.* 



In 1897, Mennes 19 showed that the immunity of guinea 

 pigs inoculated with toxins or cultures of pneumococci depends 

 upon a modification of their serum whereby an active phago- 

 cytosis is produced and that this is not due to any special 

 activity of the leucocytes. Wright and Douglas 20 introduced 

 the word opsonin (opsono, to prepare food for) to characterize 

 the substance in normal blood which they believed prepared the 

 bacteria for ingestion by the phagocytes. They were able to 

 show that this substance exists in the serum and that it is 

 partly thermolabile. It acts on the bacteria and not on the 



* These authors found in the study of phagocytosis that the leu- 

 cocytes in an immunized rabbit obtained their power of engulfing the 

 bacteria from some other substance than the serum. The immunized 

 animal fights the bacteria, first, by the direct action of the serum, 

 second, by its leucocytosis. Their conclusions were as follows: 



"(a) The serum- of normal rabbits exercises no bactericidal ac- 

 tion on the Streptococcus pyogenes. 



"(b) The serum of a rabbit on whom a certain degree of im- 

 munity to streptococcus has been conferred by vaccination, will 

 delay the development of streptococcus for several hours and will 

 sometimes exhibit a true bactericidal power. 



"(c) Leucocytes from a normal rabbit added to the serum of a 

 normal rabbit exhibit only a feeble phagocytic power to streptococcus. 

 The leucocytes die before their usual term of life. 



"(d) Leucocytes from a normal rabbit added to the serum of a 

 vaccinated rabbit energetically destroy streptococcus. The leuco- 

 cytes preserve their normal duration of life. 



"(e) Leucocytes from a vaccinated rabbit added to its own serum 

 act the same as above (d). 



"(f) Leucocytes from a vaccinated rabbit added to the serum of 

 a normal rabbit act as above (c). 



"(g) If a dose of streptococcus, lethal for a normal rabbit, is 

 injected into the pleura of a vaccinated rabbit, the serum prevents 

 the development of a pleurisy. 



"(h) If a dose of streptococcus capable of producing erysipelas 

 in a normal rabbit is injected under the skin of a vaccinated rabbit 

 the infection is prevented especially by leucocytes." 



19 Mennes. Zeit. f. Hyg., Bd. XXV (1897) p. 413. 



20 Wright and Douglas. Proc. of the Royal Society, Vol. LXXII 

 (1904) p. 357. 



