554 IMMUNITY 



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. The establishment of immunity is 

 attended by the appearance of anti-substances in the serum, and 

 the molecules of the bacteria or toxins which lead to the develop- 

 ment of these are called antigens. 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. 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 ; that is, the anti- 

 substances developed by active immunisation may be transferred 

 to a fresh animal. 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. 



Aetificial 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. 



1. By injection of the living organisms. 



(a) Attenuated in various ways. Examples : — 



(1) By growing in the presence of oxygen, or in a 

 current of air. 



