CHAPTER VI. 
IMMUNITY AND INFECTION, 
General Phenomena of Immunity. 
I. Classification of Immunity. 
A. Natural or Inherited Immun- 
ity. 
1. Racial. 
2. Indi\adual. 
B. Acquired or Induced Immun- 
ity. 
1. Active Immunity. 
(a) Natural Acquired 
Immunity. 
(6) Artificial Acquired 
Immunity. 
2. Passive Immunity. 
III. 
(a) Antibody Immun- 
ity. 
(6) Chemotherapy. 
3. Mixed, Active and Pas- 
sive Immunity. 
Infection — Primary and Secon- 
dary. 
A. Defenses of the Host, Non- 
specific and Specific. 
Theories of Immunity. 
A. The Humoral, Side-chain or 
Ehrlich Theory of Immun- 
ity. 
B. The MetchnikofT or Phago- 
cytic Theory of Immunity. 
GENERAL PHENOMENA OF IMMUNITY. i 
It has long been recognized that man and animals exhibit refrac- 
toriness to infection with specific bacteria or other microorganisms 
which cause serious epizootics in closely related animals. Man is, as 
a rule, quite free from the epizootic diseases of animals domesticated 
by him, and the domestic animals are usually not infected with the 
organisms which incite progressive disease in man. Thus, man does 
not contract chicken cholera, and domestic animals do not become 
infected with the typhoid bacillus. Furthermore, closely related 
animal species may exhibit striking differences in susceptibility to 
the same disease; for example, field mice are readily infected with 
the glanders bacillus, but house mice are quite resistant to infection 
with this organism; and ordinary sheep readily succumb to anthrax 
although Algerian sheep are practically immune to infection with 
the anthrax bacillus. 
This inherent or congenital resistance or refractoriness to infection 
with a specific microorganism, when general among the individuals 
of a species or group of animals or of man is termed natural or inherited 
immunity. It is not necessarily absolute; lowering the natural 
resistance of the individual may render him susceptible to infection. 
Thus hunger, experimental (phloridzin) diabetes, fatigue produced by 
prolonged exercise in treadmills, and excessive chilling by the removal 
of hair have been shown to decrease resistance in experimental animals. 
It is also a matter of common observation that the uniform exposure 
of a number of theoretically susceptible individuals of the same species 
to a virus does not lead to uniform infection; a certain small number 
1 An excellent and very complete discussion will be found in Zinsser's Infection and 
Resistance, .3d ed., 1923. 
