136 THE DIFFERENT TYPES OF IMMUNITY 



nity which results from the transference of protecting substances 

 from an immune animal to a non-immune individual is designated 

 as passive immunity. The difference between the two is well illus- 

 trated, if we compare the recovery of a diphtheria patient without 

 treatment with the recovery of one which follows as a consequence 

 of the administration of antitoxin. 



In the first instance the patient recovers because he succeeds in 

 forming enough antitoxin in his own body to neutralize the toxin 

 produced by the invading organism, pending the destruction of the 

 bacteria by other means, while in the second the patient is protected 

 against the deleterious effects of the toxin through the introduction 

 of the corresponding antitoxin from without. The possibility of 

 passive immunization is, of course, of the utmost importance, as 

 successful serum therapy during the actual progress of a disease is 

 dependent upon this principle, while active immunization in the 

 nature of things forms the basis of prophylactic vaccination. 



Mechanism of Different Types of Immunity. If now w r e turn to a 

 consideration of the mechanism which underlies the different types 

 of immunity, as just outlined, various possibilities suggest themselves. 



Aside from those factors which render the actual invasion difficult 

 if not impossible, such as the character of the epithelial covering 

 and the nature of the secretions which are poured out upon the 

 epithelial surfaces, the chemical and physical characteristics of the 

 medium in which the organism finds itself after invasion has taken 

 place are of necessity determinative for the question whether infec- 

 tion will or will not occur. These factors, of course, may be entirely 

 independent of any direct bactericidal action of the body cells and 

 juices per se, and have to do simply with the character of the environ- 

 ment, viewed as a culture medium for the organism in question. 



We know that certain organisms can develop successfully outside 

 of the body only, if the temperature, the reaction of the culture 

 medium, and its chemical composition are of a definite character. 

 The remarkable fastidiousness in this respect of such organisms as 

 the gonococcus and the influenza bacillus is well known. It is accord- 

 ingly quite conceivable that infection with certain organisms cannot 

 occur because of the unfavorable character of some factors of this 

 order within the body. The effect of temperature in this respect 

 is thus well shown in the case of cold-blooded animals, like the frog, 

 which is naturally immune to infection with the anthrax bacillus, 



