CHAPTER XII 



DEFENSIVE FACTOKS OF THE ANIMAL ORGANISM 



GENERAL CONSIDERATIONS 



WE have seen that the mere entrance of a pathogenic microor- 

 ganism into the human or animal body through a breach in the con- 

 tinuity of the mechanical defenses of skin or mucosa does not neces- 

 sarily lead to the development of an infection. The opportunities for 

 such an invasion are so numerous, and the contact of members of 

 the animal kingdom with the germs of disease is so constant, that if 

 this were the case, sooner or later all would succumb. It is plain, 

 therefore, that the animal body must possess more subtle means of 

 defense, by virtue of which pathogenic germs are, even after their 

 entrance into the tissues and fluids, disposed of, or at least prevented 

 from proliferating and elaborating their poisons. The power which 

 enables the body to accomplish this is spoken of as resistance. When 

 this resistance, which in some degree is common to all members of 

 the animal kingdom, is especially marked, it is spoken of as " im- 

 munity. ' ' 



From this it follows naturally that the terms resistance and im- 

 munity, as well as their converse, susceptibility, are relative and not 

 absolute terms. Degrees of resistance exist, which are determined 

 to a certain extent by individual, racial, or species peculiarities ; and 

 persons or animals are spoken of as immune when they are unaffected 

 by an exposure or an inoculation to which the normal average in- 

 dividual of the same species would ordinarily succumb. The word 

 does not imply, however, that these individuals could not be infected 

 with unusually virulent or large doses, or under particularly unfavor- 

 able circumstances. Thus, birds, while immune against the ordinary 

 dangers of tetanus bacilli, may be killed by experimental inoculations 

 with very large doses of tetanus toxin. 1 Similarly, Pasteur rendered 



'Quoted from Abel, Kolle und Wassermann, "Handbuch," etc. 



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