BENZENE RING 73 



scarlatina, typhoid fever, smallpox. The same is true of animals 

 after once having had hoof-and-mouth disease, rinderpest, and dis- 

 temper (in dogs). This is called a natural acquired immunity. 



Immunity may also be acquired by artificial means. When an 

 animal is inoculated with an attenuated virus or vaccine the aim is 

 to produce a comparatively mild attack of the disease. This mild 

 attack protects the animal for some time against another attack of 

 the same infection, be it severe or otherwise. This procedure, known 

 as (artificial) active immunity, has made the animal immune. 

 Example, anthrax vaccination with the attenuated virus. 



Passive Immunity. Besides vaccination, immunity may also be 

 conferred by injecting into an animal the antitoxic or immune serum 

 of another animal. This type is called passive immunity, and is 

 largely used in preventing tetanus or diphtheria by the subcutaneous 

 injection of tetanus and diphtheria antitoxins. 



Simultaneous Method. In some cases an active immunity is pro- 

 duced by the so-called simultaneous method, that is, by an injection 

 of a virus or vaccine and an antitoxin at the same time. An active 

 immunity generally lasts much longer and protects much better than 

 a passive immunity, but in some cases it is quite dangerous to inject 

 even a very small but still effective dose of a virus. Therefore, in 

 order to lessen the danger of severe sickness and death the antitoxin 

 is injected at the same time. The simultaneous method is employed 

 in the following cases: In inoculating horses for tetanus with both 

 tetanus toxin and antitoxin; in immunizing cattle against rinder- 

 pest by injecting simultaneously blood from a virulent case and serum 

 from an animal that has recovered from the disease; by immunizing 

 cattle against anthrax by injecting at the same time an attenuated 

 bacillus and the immune serum from a mule. This method is also 

 used in immunizing swine against hog cholera. 



Ehrlich's Side-chain Theory of Immunity. It has previously 

 been stated that toxin and antitoxin apparently unite inside and 

 outside of the body, neutralizing each other somewhat like ordi- 

 nary chemicals, such as acids and alkalies. It can be shown that 

 the experimental union of toxin and antitoxin always takes place 

 in definite amounts, and that it occurs more quickly in concen- 

 trated solutions and at higher temperatures (35 to 40 C.). These 

 facts led Ehrlich to suspect that the chemically highly complex 

 toxins and antitoxins possess certain molecular groups having toward 

 each other a high degree of chemical affinity, which causes them to 

 unite somewhat as acids and alkalies do whenever there is a chance 

 for a union. From this fundamental idea Ehrlich developed his 

 celebrated side-chain theory of immunity. In order to understand 

 this it is well first to explain what is meant by a side-chain in organic 

 chemistry. 



Benzene Ring. According to the hypothesis of Kekule, the organic 

 compounds of the aromatic group of which benzene or benzole is a 



