SCIENTIFIC BASIS OF SERUM THEORY 325 



(1) Immunity is a protective power against the invasion of the 

 body by the germs of a certain disease. 



(2) Natural immunity is a protective influence against the germs 

 of certain diseases possessed as a natural inheritance by certain 

 species of animals. An immunity, to be classed as a natural one, 

 must be possessed by all the animals of that species. For example, 

 the ox is naturally immune to hog-cholera. The hog is naturally 

 immune to small-pox. 



(3) An hereditary immunity is one that is conferred on the 

 animal before birth as an inheritance from one or both parents. 

 It differs from a natural immunity in that it is not possessed by all 

 the animals of this same species. As a rule, it is the result of the 

 mother or father of the animal having passed through an actual 

 attack of the disease. For example, pigs born of mothers that have 

 been through hog-cholera are often temporarily immune to the 

 disease. Hereditary immunity is often only temporary in charac- 

 ter and disappears as the animal gets older. 



(4) Congenital immunity is that form of protection which the 

 animal acquires as a result of passing through an attack of the 

 disease while still in the womb of the mother. It is a compara- 

 tively rare form of immunity. 



(5) By acquired immunitj' is understood an immunity or pro- 

 tective power acquired after the birth of the animal. The most 

 common way of obtaining an acquired immunity is by an attack of 

 the disease in either a mild or natural form. 



(6) Acquired inmiunity is of two principal types, active and 

 passive. 



(7) Active acquired immunity is an immunity obtained as a 

 result of an actual attack of the disease in either a severe or attenu- 

 ated form. The germs of the disease are actually present, and pro- 

 duce definite reactions on the part of the cells of the body, 



(8) This form of immunity is usually of a powerful type and 

 is permanent in nature. 



(9) Passive acquired immunity is an immunity which is con- 

 ferred as the result of injection into the body of the protected ani- 

 mal of serum from the body of an animal which possesses an aptive 

 type of immunity to this same disease. 



(10) Passive acquired immunity is usually rather temporary in 



