458 IMMUNITY 



active and passive are generally applied, or we may speak of 

 immunity directly, or indirectly, produced. We shall first give 

 an account of the established facts, and afterwards discuss some 

 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 proportionate degree of resistance or 

 immunity can thus be developed, which degree 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 consider- 

 able 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. Such a serum, generally 

 known as an anti-serum, may exert its effects if introduced 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. 



