22 VACCINE AND SERUM THERAPY. 



found a third factor of importance in immunity. This element, 

 according to these investigators, sensitizes the corresponding 

 bacteria and so modifies them that they are more readily engulfed 

 by the leucocytes. This third factor in immunity they called 

 "bacteriotropin." It is to be noticed that Neufeld and Rimpau 

 worked with virulent streptococci and pneumococci. These or- 

 ganisms are not ingested by the leucocytes in a mixture of leuco- 

 cytes, virulent streptococci or pneumococci and normal serum. 

 However, when, instead of normal serum, a specific anti-serum 

 takes part in the mixture there is ingest ion even of cultures of 

 virulent strains of these organisms. They furthermore showed 

 that the serum does not act on the leucocytes but exerts its in- 

 fluence on the bacteria, sensitizing them to the action of leuco- 

 cytes. 



There seems now to be but little doubt that opsonin and 

 bacteriotropin are the same substances, the greatest difference 

 that can be assigned, being that bacteriotropins are probably 

 what Wright and Douglas have called "immune opsonins." 



Most investigators now take an intermediate position on the 

 phagocytic and humeral theories of antibacterial immunity. It is 

 quite generally accepted that both serum and leucocytes contain 

 substances, which, acting after the manner of ferments, are able 

 to dissolve bacteria. Just as all important manifestations of life 

 are found in the normal and pathological cellular elements, so also 

 the means of defense against harmful agents is probably closely 

 related to the condition and functions of cells which prepare and 

 secrete the protective substances by means of which bacteria and 

 other harmful agents are destroyed or neutralized. 



