116 IMMUNITY AND INFECTION 
I. CLASSIFICATION OF IMMUNITY. 
The terms immunity and susceptibility are both relative; there is 
probably neither absolute immunity nor complete susceptibility to 
any infection. There is furthermore, no hard and sharp line of 
demarcation between the various types of immunity; nevertheless, 
it is convenient for purposes of discussion to assemble the prominent 
manifestations of immunity into several types or classes. 
A. Natural or Inherited Immunity. The inherited power of resisting 
specific infection manifested by a large proportion of the individuals 
comprising a family, genus or species is termed inherited or natural 
immunity. It may be: 
1. Racial.— Observed in specific families, genera or species of the 
animal kingdom. 
2. Individual.— Observed in individuals of the same species. Indi- 
vidual natural immunity may also be sexual— observed in males or 
females of the same species. 
B. Acquired or Induced Immunity.— The resistance or non-suscepti- 
bility to infection following naturally accfuired or artificially induced 
specific diseases, or resistance passively brought about by the introduc- 
tion of specific protective substances is termed acquired or induced 
immunity. 
1. Active Acquired Immunity. — (a) Natural. — FoWow'mg naturally 
acquired disease, as for example, immunity following recovery from 
smallpox or typhoid fever. 
(b) Artificial.— Brought about by the introduction of attenuated 
or killed viruses, vaccines or toxic products of bacteria into a sus- 
ceptible host. The toxic products of bacteria may be either those 
excreted during life, or products arising from their disintegration. 
Immunity to smallpox following vaccination, and immunity to typhoid 
fever following the injection of killed cultures of typhoid bacilli (vac- 
cines) are familiar examples of this type of immunity. 
There is usually a period of increased susceptibility to infection 
following immediately after the introduction of the virus or its products, 
in artificially acquired immunity. This period of susceptibility is 
succeeded by a specific increase in resistance to the virus. If the 
process of immunization is repeated several times, the initial level of 
resistance to infection may be raised very materially. Thus, prophy- 
lactic vaccination with killed typhoid bacilli (anti-typhoid vaccina- 
tion) increases the resistance of the recipient of the vaccine to typhoid 
infection to such a degree that his chances of acquiring the disease are 
greatly lessened. It is also probable that in the event of infection of 
the protected individual with the typhoid bacillus, both the duration 
and severity of the attack will be diminished. 
2. Passive Immunity. — (a) Antibody I mniimity. — Introduction into 
the host of s{)ecific products of immunity (antibodies), as diphtheria 
antitoxin. 
