NATURAL IMMUNITY 131 



a break in the continuity of the epithelial lining of the outer or inner 

 surfaces of the body occurs A A surface bruise may be followed by 

 the formation of an abscess, 'an injury to the nose may lead to 

 meningitis, the irritation of the gall-bladder by a calculus may be 

 followed by cholecystitis. The gardener or the stable man may 

 have his hands soiled by material containing tetanus bacilli with- 

 out any harm, while the infliction of a trifling wound may lead to 

 fatal lockjaw. Evidently the immunity to certain diseases which 

 one would infer from the presence of the corresponding pathogenic 

 organisms on the surface of the body in the absence of symptoms 

 of disease is only apparent. 



All that we can infer from such observations is that the surface 

 epithelium shows a certain degree of resistance to infection, i. e., 

 that in a certain sense at least it is immune. Numerous observa- 

 tions go to show, as a matter of fact, that local conditions play an 

 important role in determining the degree of resistance to infection. 

 It has thus been demonstrated that certain organisms can only 

 infect when they are introduced in a certain manner, while others 

 can do so from practically any point. The pathogenic cocci and 

 plague bacillus are examples of the latter kind, while the dysentery 

 bacillus, the cholera vibrio, and certain meat-poisoning bacilli require 

 a special portal of entry. We may then conclude that the body 

 possesses virtually no tissue immunity toward the first and a fairly 

 high grade of immunity toward the second order. Quite in accord- 

 ance with these observations is the fact that in certain animals 

 tetanus can be produced only by intracerebral injection of the 

 corresponding toxin, while in others the disease develops, no matter 

 what the character of the tissue may be in which the injection is 

 made. 



The same point is also well illustrated by the remarkable predi- 

 lection which certain organisms have for certain organs, when once 

 they have passed the outer epithelial barriers. If young rabbits 

 are thus injected intravenously with cholera vibrios they die after a 

 few days, and postmortem the organisms are found in large num- 

 bers in the intestinal mucosa, while the blood and remaining organs 

 are sterile (providing, of course, that the number injected has not 

 been unduly large.) Evidently the intestinal mucous membrane 

 offers little or no resistance to the cholera vibrio, while the other 

 tissues show a considerable degree of immunity. Well known, also, 



