272 BIOLOGY OF SPECIALIZED GROUPS 



injection into the body of the modified and attenuated bac- 

 teria or virus of a disease so as to produce the disease in a much 

 less severe form than normally. In active immunity there is 

 a reaction on the part of the body cells, while in passive 

 immunity the cells do not react and the antibody exists in- 

 active until combined with the toxic substances of the micro- 

 organisms. Acquired immunity to certain infections may, 

 to some extent, be inherited according to the latest work on 

 this subject. 



Immunity may be either antibacterial, that is, against 

 bacteria (endo toxins) , or antitoxic, that is, against the toxins 

 secreted by the bacteria. The term immunity is a relative 

 one. Large numbers of pathogenic microorganisms, or large 

 amounts of toxin, always produce some effect on the animal 

 body when injected for the purpose of experiment. 



The Theories of Immunity. (i) Exhaustion Theory. 

 It was claimed at one time by Pasteur that in acquired im- 

 munity the bacteria or infecting agents present in one infec- 

 tion used up all the pabulum necessary for the vital activity of 

 the organisms, and that in natural immunity these substances 

 were never present. Infection is, therefore, impossible. 



(2) Noxious Retention Theory. This theory held that 

 some substances of a noxious character were present in the 

 body normally in some cases, or were retained in the body as 

 a result of a previous infection. A second infection was, 

 therefore, impossible for this reason. 



(3) Phagocytic Theory. Metchnikoff claims that the 



