IMMUNITY 17 



1. By injection of the virulent organisms in non- lethal 

 doses. 



2. By injection of the dead organisms. 



3. By injection of the toxic products prepared from 

 filtered broth cultures of the organism. 



4. By the injection of the living, but attenuated, organ- 

 ism prepared by one of the undermentioned methods: 



(a) By passing through a naturally resistant animal. 

 (6) By growth at unsuitable temperatures or in unsuit- 

 able atmosphere. 



(c) By frequent and prolonged subculturing. 



(d) By growth in the presence of very weak antiseptics. 



(e) Growth in a medium of unfavourable composition 

 or reaction. 



5. By feeding dead cultures of an organism. Achieve- 

 ment of a degree of immunity by this means is slower and 

 less certain than by others. It has been more successful 

 with glucosides such as ricin and abrin, and with snake 

 venom. 



An immunised animal is said to be ' protected ' against 

 the specific disease. Protection against one disease some- 

 times also carries an immunity, though to a less degree, 

 against another. 



Passive Immunity. The serum of a protected animal 

 has an antagonistic effect on the virulent bacteria if in- 

 jected into a second animal at the same time as, or shortly 

 after, infection. Such immunity is transient. The serum 

 of an animal highly immunised against a particular toxin 

 is properly known as ' antitoxic serum ' ; that of an animal 

 highly protected against a particular organism in a virulent 

 condition is known as ' antimicrobic,' or ' antiserum.' 



To account for acquired immunity the following theories 

 have been adduced: 



Exhaustion or Pabulum Hypothesis. The bacteria are 

 assumed to abstract from the blood some compound 

 necessary for their growth, so that, once this pabulum is 

 exhausted, a second attack cannot take place until it has 

 been re-formed. As an organism will grow in the blood 

 or tissues removed from an animal immune against it, 

 this theory is untenable. Ehrlich, in his Atrepsy Theory 

 modifies the hypothesis by assuming the presence of 

 ' chemo-receptors ' for binding the poison before it can act. 



The Antidote or Retention Hypothesis. After the first 



