ARTIFICIAL IMMUNITY. 461 



Active immunity is obtained by (a) injections of the 

 organisms either in an attenuated condition or in sub-lethal 

 doses, or (I)) by sub-lethal doses of their products, i.e., of 

 their "toxines," the word being used in the widest sense. 

 By repeated injections at suitable intervals the dose of 

 organisms or of the products can be gradually increased, 

 and a proportionate degree of resistance or immunity can 

 be developed, which degree in course of time may reach a 

 very high level. Such a method can be preventive, but it 

 can never be curative, as the immunity must be developed 

 before the onset of the disease. Immunity of this kind is 

 comparatively slowly produced and lasts a considerable 

 time, the duration varying in different cases. 



Passive immunity depends upon the fact that if an 

 animal be immunised to a very high degree by the previous 

 method, its serum may have distinctly antagonistic or 

 neutralising effects when injected into another animal along 

 with the organisms, or with their products, as the case may 

 be. Here the serum of the highly-immunised animal may 

 confer immunity on another animal, if introduced at the 

 same time as infection occurs or even a short time after- 

 wards ; the method can, therefore, be employed as a 

 curative agent. The serum is also preventive, i.e., protects 

 an animal from subsequent infection, but the immunity 

 thus conferred lasts a comparatively short time. These 

 facts form the basis of serum therapeutics. When such a 

 serum has the power of neutralising a toxine it is called 

 antitoxic ; when, with little or no antitoxic power it pro- 

 tects against the living bacterium in a virulent condition it 

 is called antimicrobic or antibacterial (vide infra}. 



In the accompanying table a sketch of the chief methods 

 by which an immunity may be artificially produced is given. 

 It has been arranged for purposes of convenience and to 

 aid subsequent description, and it is not to be inferred that 

 all the different methods imply essentially different principles. 

 There is still some doubt as regards the relation of A 2, for 

 example, to A i and A 3. 



