326 DISEASES OF SWINE 



nature, and passes off as soon as the protective materials injected 

 have been entirely eliminated from the body. 



Ehrlich's Explanation of Immunity. — Just why an animal that 

 has been through an attack of a certain disease should no longer be 

 susceptible to the attack of the germs of this same disease is rather 

 hard to understand. It is easy to see that this is a wise provision on 

 the part of mother nature, but, like many other of mother nature's 

 simple workings, it has long been a mystery to the keenest of 

 human minds how this state of affairs is brought about. Professor 

 Ehrlich has devoted years of study to the subject, and his explana- 

 tion of the reactions that take place to cause the development of 

 immunity are perhaps the most acceptable of any theories that 

 have as yet been advanced. Ehrlich's explanation, which is 

 known in scientific works as Ehrlich's side-chain theory, is sub- 

 stantially as follows: 



Every cell in the animal body must have nourishment. This 

 nourishment is necessary in order that the cell may live, grow, and 

 perform its allotted share of the daily work of the body. For the 

 purpose of receiving this nourishment each cell is provided with 

 small receiving arms, which are known as the receptors of the cell. 

 In the normal healthy cell there is only one of these receptors, and 

 if anything happens to disable or damage this receptor the cell must 

 form another one to replace it before it is able to continue its daily 

 work and keep up its natural growth and repair. 



When the germs of a disease enter the body of an animal they 

 are about to attack they at once commence to manufacture certain 

 very poisonous materials which are known as toxins. The toxin 

 is the weapon of attack of the germ, and it is with these toxins that 

 they hope to destroy the c'fells of the animal body and become the 

 victors in the battle that is about to take place between the cells 

 of the body and the invading army of germs. After all, this is all 

 that disease of any kind consists of. All disease is a battle between 

 the cells of the body and the cells of disease, which are known as 

 the germs. For instance, when the germs of pneumonia enter the 

 body of man or animal they begin an attack, and this battle between 

 cells and germs is kept up for many days. In some cases the cells 

 win the battle, and then the patient gets well. In other cases the 

 germs are returned the victor, and then the patient dies. The 



