8 IMMUNE SERA. 



gradually increase the dose injected, it will be pos- 

 sible after a time to inject at one dose an amount 

 of cholera bacilli that represents many times an 

 ordinary fatal dose. If from this animal we now 

 withdraw serum and inject it into another animal, 

 we find that this serum, even in such small amounts 

 as the fractional part of a centigram or even of a 

 milligram, is able to protect the second animal 

 against living cholera bacilli. Under the influence 

 of these small amounts of serum of the treated ani- 

 mal, the organism of the untreated animal is able 

 to dissolve large amounts of cholera bacilli, amounts 

 which would otherwise be invariably fatal. This 

 process, as R. Pfeiffer showed, is a specific one, i.e., 

 the serum of the guinea-pig treated with cholera 

 bacilli transmits an increased solvent power only 

 for cholera bacilli, but not for any other species of 

 bacteria. The active substance of such a bacterio- 

 lytic immune serum Pfeiffer called a specific bac- 

 tericide. If we allow some of this specific cholera 

 immune serum to remain for some time outside of 

 the body, e.g. in a bottle, and then test it for 

 solvent properties against cholera bacilli, not in a 

 living body but in a test-tube, we shall find that its 

 power is almost nil. If we add to this serum in 

 the test-tube some fresh peritoneal exudate or 

 some other body fluid, such as serum of a normal, 

 untreated guinea-pig, as Metchnikoff first did, we 

 find that this serum has now acquired the power 

 to rapidly dissolve cholera bacilli even in a test-tube. 



