IMMUNITY 71 



Types of Immunity. Immunity is classified as (1) 

 natural or racial or species immunity, and (2) acquired 

 immunity, which latter has been further divided into 

 active and passive. 



Natural immunity is the condition wherein a certain 

 disease does not occur in the type of animal under 

 consideration; as, for example, the dog does not take 

 typhoid fever even when fed a pure culture of the 

 specific germs. There is also a relative natural im- 

 munity. Cats present great resistance to infection 

 with anthrax. 



Racial immunity is shown by great resistance of the 

 negro to yellow fever. 



There is also individual immunity, as shown by the 

 passing of a person through a virulent epidemic without 

 the slightest sign of illness. 



Acquired immunity is that resistance which a 

 person obtains by passing through an attack of disease. 

 That a second attack of measles or scarlatina seldom 

 occurs is well known. This is seen also in typhoid 

 fever. Such an acquired immunity is called active 

 acquired immunity because the economy has had to 

 work for its own protection, and it is only good for 

 the one kind of disease, supplying no protection to 

 any other kind: that is, it is specific. There is also 

 a passive acquired immunity, by which is meant that 

 some protective substances from another individual 

 are added to the natural resistance of the body. This 

 passive acquired immunity is very well shown in diph- 

 theria when the serum of a horse which has been 

 rendered resistant to the toxin of the diphtheria bacilli 

 is given to the patient. This horse is said to possess 



