THtORIES OF IMMUNITY. 



this case it may be spoken of as an artificial 

 immunity, and such artificial immunity has been 

 obtained in a number of different ways. 



Acquired immunity, as seen following- the 

 action of bacterial infectious agents, is again 

 divided into the forms of Active and Passive 

 Immunity, and the distinction between these 

 two is of great importance in understanding 

 many of the problems arising in connection 

 with the study of immunity in general. 



Active acquired immunity occurs in the course 

 of recovery from an infection, or may be pro- 

 duced artificially. In the latter case it is the 

 result of the injection of gradually increasing 

 doses (amounts) of the toxic products of micro- 

 organisms, beginning with quantities much less 

 than the minimal lethal dose, and increasing until 

 there is reached a condition of resistance without 

 reaction to many times such minimal lethal dose. 

 Such a condition may be obtained by the injec- 

 tion of the bodies of bacteria killed by the appli- 

 cation of heat; of the bodies of bacteria in the 

 living condition, but whose virulence has been 

 diminished in various ways; or by the use of 



