44 8 IMMUNITY. 



able potency, which may act against either of these two 

 factors. In the first place, a serum may protect against the 

 separated toxine, or, in other words, may be antitoxic. 

 In this case there is immunity also against the living 

 organisms, as might naturally be expected ; for when their 

 toxines are neutralised their harmful action on the 

 tissues is removed, and they are then destroyed probably 

 by the same means as ordinary non-pathogenic organisms. 

 In the second place, a serum may lead to the destruction 

 of the organisms. In this case, it is usually indirectly 

 bactericidal, i.e., becomes bactericidal in certain conditions, 

 though in many instances a directly paralysing action on the 

 organisms has also been demonstrated. The term anti- 

 microbic is, therefore, conveniently applied to such a serum. 

 In many instances an antimicrobic serum has little or no 

 effect against the toxines ; this is the case with the anti- 

 streptococcic serum (Marmorek), the anti-cholera and anti- 

 typhoid sera (Pfeiffer), and many others. The action of 

 both varieties of anti-sera, so far as is known, is specific, 

 being exerted only against the particular organism or 

 toxine by the action of which the immunity has been 

 produced. In the case of both, immunity can be trans- 

 ferred to another animal by means of a certain quantity of 

 the serum, the latter having a definite value which can be 

 ascertained by experiment. It does not follow from what 

 has been said that a serum may not act in both of the 

 ways described. A given serum might, for example, be 

 powerfully antimicrobic and feebly antitoxic at the same 

 time, and even a single chemical substance might conceiv- 

 ably have both effects. Nevertheless the two modes of 

 action are distinct. 



Theories as to Acquired Immunity. 



The advances made within recent years in our know- 

 ledge regarding artificial immunity and the methods by 

 which -it may be produced have demonstrated the in- 

 sufficiency of various theories which had been propounded. 



