CHAPTER VII 

 ANTIGENS AND ANTIBODIES 



\YE have seen in the foregoing chapters that the normal animal 

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

 fully meet a developing infection, with certain organisms at least, 

 providing that the invading numbers are not too large. In labora- 

 tory parlance we express this by saying that successful resistance is 

 possible, if the bacterial dose falls short of the minimal infecting 

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

 amount. 1 



If now we compare the bacteriolytic titre 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 peritoneal 

 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 noted that at a cer- 

 tain 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 synonymously 

 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. 



