324 DISEASES OF SWINE 



mains throughout the natural hfe of the animal. This is due to the 

 fact that the cells of the body have been trained in the art of germ 

 defence, and they never lose the power of keeping a constant guard 

 against attack by these same germs. 



Passive Immunity. — It has been found by experiments that 

 have been made on a large number of animals that the protective 

 power of immunity which an animal that has been through an 

 attack of disease acquires thereby is in large measure due to the 

 presence of some new material in the blood of the animal. It has 

 also been found that if the blood be drawn from the body of this 

 animal and injected into the body of another animal it will protect 

 this second animal against an attack by these same germs. In this 

 latter case the protected individual acquires an immunity against 

 the disease in question without any activity on his part whatever. 

 The protected animal in this case is entirely inactive or passive, as 

 far as developing any protective bodies is concerned. This type 

 of immunity is, accordingly, known as a passive immunity. 



Passive immunity differs from an active immunity in that it 

 is not the result of an attack, either severe or mild, of the disease 

 protected against. In active immunity there is always an actual 

 attack by the germs of the disease, either in their full virulent 

 power or in attenuated form. In passive immunity the animal 

 acquiring this sort of protection merely allows the blood of an 

 actively immune animal to resist the invasion of the germs. There 

 is also a marked difference in the duration of the protection or 

 length of time which it lasts. In active immunity, as has already 

 been stated, the protection often lasts throughout the remainder 

 of the lifetime of the animal. In passive immunity this is not the 

 case. When the protecting blood is injected into the body of the 

 susceptible animal it gives protection only so long as it remains in 

 the body of the injected animal. This is only a comparatively 

 short period — a few weeks at most. In the case of hog-cholera the 

 average duration of the passive immunity given by injection of 

 hog-cholera serum, when given by the single method, is six to ten 

 weeks. 



Summary of Immunity.— From the foregoing paragraphs we 

 are able to arrive at the following conclusions concerning the sub- 

 ject of immunity: 



