PHAGOCYTOSIS 47 



individual may contract a disease, while another exposed at the 

 same time will not. Inherited immunity is exemplified by the 

 history of races into which a new disease was introduced at first 

 with high mortality but later with great reduction in the severity 

 of the infection. 



Acquired Immunity. Actively acquired by infection. One 

 attack of yellow fever immunizes the individual against subse- 

 quent attacks. Vaccination actively immunizes against small-pox. 



Passively Acquired. Actually injecting protective substances 

 (anti-toxic sera) into the blood. The immunity againsta 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, the oldest ones, beginning with 

 Pasteur, being that some substances vitally necessary to the 

 virus was used up or that something was retained to affect new 

 microbial attacks. 



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 

 ingenious 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- 

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

 cells, defend the body against infection by devouring the in- 

 vading agents of disease (Fig. 14). 



Metchnikoff considers the subject of phagocytosis under three 

 aspects; (i) nutritional; (2) resorptive; (3) protective. 



