426 IMMUNITY. 



description of the methods, and discuss the "principles 

 involved. According to the means by which it is produced, 

 immunity may be said to be of two kinds, to which the 

 terms active and passive are generally applied, or we may 

 speak of immunity directly, or indirectly, produced. 



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 " 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. In this method a series of reactions is 

 developed within the animal, and this leads to immunity. 

 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 compara- 

 tively slowly produced, and lasts a considerable time, though 

 the period varies 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 has 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, and 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. 



The method of producing passive immunity was first 

 worked out in the case of tetanus and diphtheria. A high 

 degree of resistance was obtained in certain animals by 

 repeated and gradually-increasing doses of toxine separated 

 by filtration, and it was then found that their serum could 



