48 IMMUNE SERA 



suggested for it the name " alexin." He found that 

 it was possible to greatly increase the bactericidal 

 action, (i.e. the quantity of " alexin ") for a par- 

 ticular bacterium by immunizing an animal with 

 that bacterium. 



Pf eiffer's Phenomenon. An enormous advance 

 in the study of immunity was made in the dis- 

 covery of Pfeiffer's phenomenon in 1894,' and it is 

 to Pfeiffer's splendid observations * that we owe 

 the first and most important insight into the mode 

 of action of the bacteriolytic immune sera. A 

 normal guinea pig is able to kill and dissolve a 

 number of living cholera bacilli if these are in- 

 jected intraperitoneally. If in such an animal we 

 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., 



1 R. Pfeiffer, Zeitschr. Hygiene, Vol. xviii, 1894. 



