50 INFECTION AND RESISTANCE 



the external defences of skin and mucous membrane which, in the 

 healthy state, mechanically prevent the entrance of the micro-organ- 

 isms into the body. For we have seen, in another place, that few of 

 the bacteria can pass through the uninjured surfaces. Moreover, 

 added to this, there is some protection in the bactericidal properties 

 of the secretions. An example of this is the inhibitory power exer-\ 

 cised by the acidity of the normal gastric juice upon the choler^r ' 

 spirillum. In order to infect the intestinal canal of guinea pigs 

 with these organisms Koch found it necessary to neutralize the gas- 

 tric juice with sodium carbonate solutions, and other observers have 

 found it necessary to inject directly into the duodenum. But even 

 after entrance into the animal tissues a second line of defence is 

 normally encountered by all invading germs which tend to inhibit 

 their further progress more or less perfectly. This active opposition 

 to the bacteria after their entrance is expressed chiefly in the anti-\ 

 bacterial (bactericidal) activity of the blood serum, and the pha-/ 

 gocytic powers of leukocytes and other cells. To a certain extent 

 these forces are active against all bacteria in all animals, but they 

 may vary in different species, races, or even individuals in potency 

 against any given infectious agent, and, to a certain extent, varia- 

 tions in resistance may be referable to this. The analysis of these 

 forces, both in the normal and in the artificially immunized animal, 

 forms the substance of the systematic discussions which are to fol- 

 low, and, for the present, we will confine ourselves to an examination 

 of the facts that have been gathered regarding the actual differences 

 in normal resistance or "Natural Immunity" between various spe- 

 cies of animals. 



And if we glance over the list of diseases to which different spe- 

 cies and races of animals are victim, it is immediately evident that 

 some animals are never spontaneously infected with many of the) 

 micro-organisms that cause extensive and fatal ravages in others. 

 Also, within the same race or species, an epidemic sweeping through 

 a community will kill many individuals and leave others unscathed.) 

 Such differences point to variations in the defensive mechanism, 

 since the invader in these cases is the same. We speak, therefore, of 

 Natural Immunity which is an attribute of species, that which, 

 within the same species, is racial, and that which, within the same 

 race, is individual. And the attempts to discover the causes under- 

 lying such differences in natural resistance have elucidated many of 

 the fundamental principles of immunity in general. 



Instances of natural immunity which appear to depend on spe- 

 cies are common. We have pointed out, above, that in order to make 

 infection at all possible, it is necessary that the invading germ shall 

 find suitable cultural conditions in the body of the host. It is this 

 simple principle which probably explains the fact that bacteria whick 

 cause disease in warm-blooded animals cannot, as a rule, cause di/ 



