134 



INFECTION AND IMMUNITY. 



inn 



Active 



innity. 



Passive 

 Immunity. 



ticularly to habituation to various drugs, as mor- 

 phin, cocain or arsenic, to which a high resistance 

 may be acquired. The true explanation of such 

 resistance is not known, but it would appear to 

 depend on some acquired property by the cells 

 which were originally more susceptible; in other 

 words, it would appear to be an immunity which 

 is fixed in the cells, a static immunity, so to say, 

 if the expression may be coined. It may depend on 

 habituation alone, on an increased power of de- 

 stroying the poison by the cells, or on a decreased 

 affinity of the cells for the poison. 



A similar process may exist in acquired resist- 

 ance to some of the more chronic infections, as 

 tuberculosis, leprosy and others, although very lit- 

 tle is known definitely concerning it. 



Immunity which results from an infection de- 

 pends on a specific reaction on the part of the 

 tissue cells in response to the chemical injury pro- 

 duced by the bacteria or their toxins. The indica- 

 tions of the occurrence of such a reaction lie, first, 

 in the recovery of the patient, and, second, in the 

 new antitoxic or antibacterial power which may be 

 demonstrated in the serum or the increased phago- 

 cytic power of his leucocytes. In view of the active 

 part played by the body in establishing this new 

 resistance, the condition is referred to as an active 

 immunity. In the preparation of various anti- 

 toxic and antibacterial serums for therapeutic pur- 

 poses, a condition of active immunity is deliberate- 

 ly produced in the animals (the horse, for exam- 

 ple) by the injection of the toxins or of the bac- 

 teria. 



Contrariwise, the resistance which is established 

 in an individual through the injection of an im- 



