IMMUNITY AND SUSCEPTIBILITY. Ill 



cemia. This observation may explain some of the pneu- 

 monias occurring in consequence of or during the course 

 of diphtheria. 



It is not understood what the two affections may have 

 in common, but when yellow fever breaks out epidemic- 

 ally it is apt to be associated with a simultaneous epi- 

 demic of dengue fever. 



j. Traumatic injury seems to increase susceptibility, 

 probably by providing a nidus in which the bacteria may 

 develop free from the usual restraints. Attenuated cult- 

 ures of the bacillus of symptomatic anthrax which would 

 not kill a guinea-pig, may do so if simultaneously intro- 

 duced into the tissues with a little lactic acid. Vaillard 

 and Rouget 1 found that if tetanus bacilli were introduced 

 into the body, washed free from their toxin, they were 

 readily taken up by the phagocytes and no signs of dis- 

 ease followed. If, however, their toxin, some lactic acid 

 or other damaging cheinic substance, were introduced 

 with them and the cells of the tissue destroyed, they 

 grew and caused tetanus. 



II. Exaltation of Immunity. — By means which 

 rarely if ever arise under natural conditions it is pos- 

 sible to intensify the natural immunity possessed by an 

 animal. As, however, the process is identical with the 

 development of immunity where none formerly existed, 

 and is a purely artificial condition, it finds its best con- 

 sideration below. 



Acquired Immunity. 



Acquired immunity is power to resist disease, depend- 

 ing upon conditions arising during the life of the indi- 

 vidual. It is a peculiarity of an individual, not of the 

 species. In general, the condition resembles natural im- 

 munity, but in a few particulars it is very different, and 

 is characterized by certain peculiar phenomena. In the 

 arrangement of the subject, as considered below, acciden- 

 tally acquired immunity will be found to bear the greatest 



1 La Bull, mid., 1891, p. 901. 



