IMMUNITY 381 



the cells of the various tissues. (3) It is generally true that 

 individuals are not equally susceptible to the same poisons and 

 natural immunity rests, in part, on this fact. The immune 

 individual is not affected by the bacterial products which act as 

 toxins in other individuals. This is accounted for by the pres- 

 ence in the blood of the immune individuals of a substance, 

 which neutralizes the toxins, and is, therefore, called antitoxin. 



777. The animal organism is not passive to the attacks of 

 bacteria. If the attack is not too sudden and violent the 

 tissues respond by producing antitoxin, which neutralizes the 

 effect of the toxin, and a lysin (alexin), which causes the dis- 

 solution of the bacteria. Another substance is also formed 

 which causes the bacteria to adhere in clumps. This is called 

 agglutinin. These substances (anti-bodies) produced by the 

 tissues in response to the bacterial stimulus, may continue to be 

 formed long after the exciting cause has disappeared and the 

 body is therefore immune to a second attack. This is known 

 as acquired immunity. 



778. Bacteria are extremely variable. This is especially 

 evident in the degree of virulence of different strains of what is 

 apparently the same species. Immunity acquired from the 

 attack of a mild form is also generally efficient against the more 

 virulent types. This principle is employed to produce immunity 

 by intentionally infecting or inoculating an individual with a 

 mild type and thereby securing protection against more danger- 

 ous forms. Immunity secured in this way is active acquired 

 immunity. 



779. Passive immunity is secured by injecting into the 

 animal a blood serum obtained from an immune animal. 

 This serum-therapy is effective at the time, but the anti- 

 bodies soon disappear from the blood and, since the tis- 

 sues have not been stimulated to the formation of anti- 

 bodies, the immunity is lost. 



780. The various types of immunity may be illustrated by 



