IMMUNITY 69 



ft 



the injected micro-organisms. We have ah^eady said 

 that fresh blood-serum is able to kill a considerable num- 

 ber of bacteria; when injected with gradually increas- 

 ing amounts of bacteria, however, this destructive power 

 increases enormously, but only for the particular kind of 

 bacterium used for injection. Thus, if an animal is 

 injected with typhoid bacilli the serum will, after a time, 

 kill enoraious numbers of typhoid bacilli, even in very 

 small doses; tested against cholera bacilli, or any other 

 bacteria, its destructive effect is merely that of normal 

 serum from an untreated animal. AMien the action of 

 the serum on the bacteria is studied under the micro- 

 scope it is seen that the bacteria are actually dissolved. 

 Hence, such a serum is spoken of as a ^^bacteriolysin," 

 which means bacteria dissolving. Since the bacteria 

 are killed by this action, w^e also speak of the serum as 

 being ' 'bactericidal,'^ which means bacteria kilhng. 



It has been found that this action of the serum may 

 be developed against other cells than bacteria. When 

 red blood-cells are used the serum acquires dissolving 

 properties for these; and here, again, the action is 

 strictly specific, so that when blood-cells from a chicken 

 are injected into an animal the serum of the injected 

 animal acquires increased solvent powers only for 

 chicken blood-cells, but not for blood-cells of other 

 animals. Instead of using the word bacteriolysin, we 

 speak of such a serum as a hemolysin, meaning blood 

 dissolving. 



The term " cytolysin " is used to embrace all these 

 dissolving sera, '' cyto " signifying cell; hence, cell 

 dissolving. 



Investigation showed that the mode of action of these 



