514 IMMUNITY 



of the theories which have been brought forward in explanation 

 of these facts. 



Active immunity is obtained by (a) injections of the organisms 

 either in an attenuated condition or in sub-lethal doses, or (b) 

 by sub-lethal doses of their products, i.e. of their "toxins," 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 ; or, what practically amounts to the 

 same, an organism of greater virulence or a toxin of greater 

 strength may be used. A degree of resistance or immunity 

 can thus be developed, and this in course of time may reach 

 a very high level. Such methods constitute the means of 

 preventive inoculation or vaccination. Immunity of this kind 

 is comparatively slowly produced and lasts a considerable time, 

 the duration varying in different cases. The principles of 

 vaccination have within recent years been extended by Wright 

 to the treatment of disease. 



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. Such a serum, 

 generally known as an anti-serum, may exert its effects if intro- 

 duced into an animal at the same time as infection occurs or 

 even a short time afterwards ; it 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 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 merely for purposes of convenience and to 

 aid subsequent description ; the principles underlying all the 

 methods are the same. 



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. 



