CLASSIFICATION OF IMMUNITY 113 



immunity to the specific infection; the host is a passive recipient of 

 antibodies in such instances. These alien antibodies, however, soon 

 diminish in potency or disappear, leaving the susceptibility of the indi- 

 vidual to infection at its original level. Immunity induced by the 

 injection of specific antibodies is termed passive acquired immunity. 

 The transitory immunity to diphtheria or tetanus following the 

 injection of diphtheria or tetanus antitoxin is an example of passive 

 acquired immunity. 



Immunity may be localized or general in the same individual, and 

 different individuals frequently exhibit varying degrees of resistance 

 or susceptibility to the same virus. 



I. CLASSIFICATION OF IMMUNITY. 



Both immunity and susceptibility are relative; there is probably 

 neither absolute immunity nor complete susceptibility to any infec- 

 tion. There is furthermore, no hard and sharp line of demarcation 

 between the various types of immunity; nevertheless, it is convenient 

 to assemble the prominent manifestations of immunity into several 

 types or classes. 



A. Natural or Inherited Immunity. The inherited power of resist- 

 ing 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, or 



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-sus- 

 ceptibility to infection following naturally acquired or artificially 

 induced specific diseases, or resistance passively brought about by the 

 introduction of specific protective substances is termed acquired or 

 induced immunity. 



1. Active Immunity. (a) Natural. Following 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 

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