CHAPTER VII. 

 ANTIGENS AND ANTIBODIES. 



WE have seen in the foregoing chapters that the normal animal 

 has a defensive mechanism at its disposal with which it may suc- 

 cessfully meet a developing infection, with certain organisms at 

 least, providing that the invading numbers are not too large. In 

 laboratory parlance we express this by saying that successful resist- 

 ance is possible, if the bacterial dose falls short of the minimal infect- 

 ing amount, or if this should be exceeded, at least of the minimal 

 fatal amount. 1 



If now we compare the bacteriolytic titer of the serum of an animal 

 that has received an injection of a subfatal dose with that of a normal 

 control, or with that which the same animal showed before the 

 injection, a remarkable increase will be noted which may be further 

 raised by additional injections. Upon then examining the peri- 

 toneal contents of a normal animal that has received a minimal 

 fatal dose and comparing the results with the findings in a second 

 animal which has been previously injected with a subfatal dose and 

 which now receives the same amount as the first, it will be found 

 that at a certain time the peritoneal fluid of the previously injected 

 animal will have become sterile, while that of the untreated control 

 is swarming with organisms; and, moreover, while the latter dies, 

 the other recovers and thus shows itself, relatively at least, immune, 

 using this term in the original sense of its meaning and synony- 

 mously with "resistant." 



This immunity was evidently produced through the activity of 

 the animal itself, and is hence appropriately spoken of as active 

 immunity in contradistinction to passive immunity, which latter 

 results when the immunity-bestowing substances that were actively 

 produced in the one animal are artificially transferred to a second 

 (normal) one. The possibility of such a transference can be readily 



1 These considerations apply essentially to infections with the so-called semi- 

 parasites, exemplified by the cholera vibrio and the typhoid bacillus. 



