CHAPTER XII 



DEFENSIVE FACTORS OF THE ANIMAL ORGANISM 

 GENERAL CONSIDERATIONS 



WE have seen that the mere entrance of a pathogenic microorganism 

 into the human or animal body through a breach in the continuity of 

 the mechanical defenses of skin or mucosa does not necessarily 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 mor subtle means of defense, by virtue of which pathogenic 

 germs are, even after their entrance into the tissues and fluids, dis- 

 posed 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 com- 

 mon to all members of the animal kingdom, is especially marked, it is 

 spoken of as "immunity." 



From this it follows naturally that the terms resistance and immunity, 

 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 ex- 

 posure or an inoculation to which the normal average individual 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 unfavorable 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 naturally immune hens 

 susceptible to anthrax by cooling them to a subnormal temperature, and 

 Canalis and Morpurgo did the same with doves by subjecting them to 

 starvation. 



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

 189 



