IMMUNITY 



247 



more susceptible to scarlet fever, measles, whooping cough, 

 etc., than adults; young cattle more frequently have black-leg 

 than older ones (these apparently greater susceptibilities may 

 be due in part to the fact that most of the older individuals 

 have had the diseases when young and are immune for this 

 reason) . Animals weakened by hunger or thirst succumb to 

 infection more readily. Frogs and chickens are immune to 

 tetanus, but if the former be put in water and warmed up to 

 and kept at about 37° C. and the latter be chilled for several 

 hours in ice-water, then each may be infected. Pneumonia 

 frequently follows exposure to cold. The immune rat may be 

 given anthrax if first he is made to run in a "squirrel cage" 

 until exhausted. Alcoholics are far less resistant to infec- 

 tion than temperate individuals. "Worry," mental anguish, 

 tend to predispose to infection. 



The following outlines summarize the different classifi- 

 cations of immunity so far as mammals are concerned for 

 the purposes of discussion. 



Immunity. 



I. Natural 



II. Artificial- 



Inherited through the germ cell or cells. 



(a) By having the 



A. Congenital \ disease in utero. 



(6) By absorption 

 of immune sub- 

 stances from the 

 mother. 



2. Acquired in uiero. 



by havinj 



B. Acquired post partu 



the disease, 

 -acquired through human agency by: 



1. Introduction of the organism or its products. 



2. Introduction of the blood serum of an immune animal 



Immunity. 



I. Active — due to the introduction of the organism or due to the intro- 

 duction of the products of the organism. 



A. Naturally by havijig the disease. 



B. Artificially. 



1. By introducing the organism: 



1. Parage through another animal. 



2. Drying. 



3. Growing at a higher temperature. 



4. Heating the cultures. 



5. Treating with chemicals. 



6. Sensitizing. 



7. Cultivation on artificial media. 



2. By introducing the products of the organism. 



11. Passive — due to the introduction of the blood serum of an actively 

 immunized animal. 



(a) Alive and virulent. 



(6) 

 (c) 



Alive and virulence 

 reduced by 

 Dead. 



