ARTIFICIAL IMMUNITY. 463 



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 com- 

 paratively 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 ani- 

 mal, if introduced at the same time as infection occurs or even a 

 short time afterwards ; 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 con- 

 ferred lasts a comparatively short time. These facts form the 

 basis of serum therapeutics. When such a serum has the power 

 of neutralising a toxin it is called antitoxic ; when, with little or 

 no antitoxic power, it protects 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 subse- 

 quent description, and it is not to be inferred that aU the differ- 

 ent 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. It will presently be seen that in the production 

 of immunity it is to be noted that the method to be chosen 

 usually depends on the individual organism against which im- 

 munity is to be conferred. Thus the injection of diphtheria 

 bacilli will immunise both against subsequent infection by bacilli 

 and against the injection of diphtheria toxin, and immunisation 

 by diphtheria toxin will have a similar effect. 



Artificial Immunity. 



A. Active Immunity — i.e. produced in an animal by an in- 

 jection, or by a series of injections, of non-lethal doses of 

 an organism or its toxins. 



