IMMUNITY 201 



rabbit's blood be first treated with the blood of a lower animal, human 

 blood will not cause the precipitate. 



A strange phenomenon has also come forth in recent years known 

 as an anaphylactic shock. This is probably connected in some way with 

 the precipitation reaction. It means that an animal already immunized 

 to a protein may die when additional protein of the same kind is in- 

 jected. This condition in an animal is known as anaphylaxis ( ). 

 In other words, we may say anaphylaxis is an oversensitiveness of the 

 organism toward bacterial toxins and foreign sera. The reason for ana- 

 phylaxis is not yet satisfactorily explained. 



The principle of anaphylaxis is used to diagnose certain diseases. 

 The tuberculin reaction is nothing more nor less than the injection of 

 the proteins of the tubercle bacillus into the skin, which (because the 

 dose is very small) does not overwhelm the entire nervous system, but 

 produces only a fall in temperature and a slight fever, if the disease is 

 present. 



An antitoxin is, as already stated, the soluble substance produced 

 which neutralizes quantitatively fresh injections of the same poison. 

 The commercial diphtheric antitoxin is merely the serum of a horse 

 which has had repeated doses of diphtheria toxin injected until it has 

 been brought into a state of active antitoxic immunity. Like all im- 

 munizing agents antitoxins are all specific for some single toxin. 



In this connection it is interesting to note that while sheep are very 

 susceptible to the toxin formed by the tubercle bacilli, they are not sus- 

 ceptible to the injection of the dead bacilli themselves. Guinea pigs are 

 quite susceptible to the bacilli but not to the toxin. The proteins of 

 one's own body injected into ourselves are poisonous. 



The various ways in which immunity is produced by injection of 

 foreign substances may be summarized as follows : 



(1) The virulent parasites are administered in small doses so as 

 to give the individual the disease in a mild form (active immunity). 



(2) Weakened parasites may be injected in larger doses and pro- 

 duce the same result. 



(3.) Dead bacteria may be given in place of living, so as to produce 

 a feebler but similar result. 



(4) The poisons may be isolated from the parasites, and gradually 

 increasing doses are injected, thus increasing the normal neutralizing 

 ability of an individual. 



(5) Serum from an animal, immunized by one of the above pro- 

 cesses, may be placed directly into another individual and thus permit 

 him to become immune without going through any form of the disease 

 himself (passive immunity). 



