THEORIES OF IMMUNITY. 



of both of these diseases being bacteria that pro- 

 duce extra-cellular or soluble toxines. 



They do not occur, or only to a very limited 

 extent, in animals attacked by or immunized 

 with the toxic products (bacteria) of typhoid 

 fever, cholera, or plague ; the cause of these dis- 

 eases being bacteria that produce intracellular 

 or non-soluble toxines. 



When they do occur and are used for pro- 

 tective purposes, the condition following their 

 use is spoken of as Passive Immunity. 



The differences between these two forms of 

 immunity are very marked. 



Active immunity is slow in appearing, is more 

 or less dangerous to secure, is always attended 

 with at least some discomfort, but is very lasting 

 when once attained. A horse brought to a high 

 degree of immunity to diphtheria toxine will 

 show a marked degree of such immunity for a 

 very long time — extending over months and 

 even years. 



Passive immunity, on the other hand, is very 

 rapid in its appearance, is attended with no 



