42 IMMUNITY 



Young adults are more susceptible to typhoid fever than are elderly 

 ones. Infants are exceedingly prone to suffer from milk infection 

 while older children are not. Certain diseases are known as chil- 

 dren's diseases, because adults rarely have them. Again, one 

 individual may contract a disease, while another exposed at the 

 same time will not. 



Acquired Immunity. Actively acquired by infection. One 

 attack of yellow fever immunizes the individual against subsequent 

 attacks. Vaccination actively immunizes against small-pox. 



Passively acquired. Actually injecting protective substances 

 (anti-toxic sera) into the blood. The immunity against a given 

 disease (diphtheria) resides in the anti-toxic sera. 



Immunity is nearly always relative. A small quantity of toxin 

 may be innocuous, while a large quantity may cause a fatal toxaemia. 



There have been several theories advanced to account for the 

 various phenomena of immunity. 



Exhaustion Theory. Pasteur conceived that bacteria as they 

 grow in the body, use up or exhaust something vitally necessary to 

 the subsequent growth of that particular kind of bacteria. 



Retention Theory. It was held that some noxious agent was 

 retained by the body which prevented the further growth of bacteria. 



The modern conception of immunity deals with two theories, 

 the theory of phagocytosis of Metchnikoff, which may be termed 

 the cellular or biologic one, and the lateral-chain, or the humoral 

 or chemical theory of Ehrlich. Both of these are extremely ingeni- 

 ous and explain satisfactorily why certain bacteria are unable to 

 infect the body, and why, the body once infected, cannot, in many 

 diseases, be again infected. Furthermore these theories make it 

 clear to us why the body tissues during life do not fall an easy 

 prey to many putrefactive bacteria, as after death. 

 / Phagocytosis is essentially a theory of cell devouring. Leuco- 

 \ytes which are white mobile cells of the blood, and other fixed cells, 

 defend the body against infection by devouring the invading agents 

 of disease. (Fig. 16.) 



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