NATURAL IMMUNITY 167 



infectious diseases, such as measles, pertussis, etc., without becoming 

 infected. I have noticed, on several occasions, that resident physicians, 

 on service in scarlet-fever wards for many months or years, having 

 escaped infection though brought in intimate contact with severe forms 

 of the disease, finally contracted the disease upon returning from a short 

 vacation. 



Natural immunity may be due to the following causes: 



1. Various non-specific factors may prevent infection; among these 

 may be mentioned (a) The integrity of the epithelium of the skin and 

 mucous membranes, and (b) the chemical and physical action of various 

 secretions, such as the gastric fluid, the intestinal juices, and the saliva. 



2. A particular route for the introduction of infecting microparasites 

 may be necessary. For example, intestinal diseases, such as typhoid 

 fever and cholera, are usually due directly to swallowing of the infecting 

 microorganisms, infection in this type of disease seldom, if ever, occurring 

 through the skin. This is probably due in part to the lowered vitality 

 of the intestinal mucosa, together with a peculiar selective affinity of 

 the bacteria for the cells of these tissues, aided by (a) the biologic 

 nature of the invading bacterium, which grows best under the more 

 favorable cultural conditions of the intestinal canal. This selective 

 action is further illustrated by the tendency of dysentery toxin to attack 

 the intestinal mucosa when the bacilli or toxin is administered intra- 

 venously. 



3. Certain tissues appear to possess a marked local immunity to certain 

 bacteria. In considering examples of local immunity, various factors, 

 such as the question of exposure, the thickness of the epidermis, and the 

 kind and quantity of the local secretions must be borne in mind. 

 For example, Trichina spiralis affects the muscles, never the bones, 

 and but rarely any other tissue. Likewise, although diphtheria in the 

 throat may spread in many directions, it seldom passes down the esopha- 

 gus. 



Some differences are known to exist in regard to local immunity as 

 observed in the child and in the adult. For example, ring-worm of the 

 scalp is practically unknown among adults, whereas children under 

 seven years of age are quite susceptible to the disease. These differences 

 may be due to the greater susceptibility in general of young tissues to 

 infection, and the local immunity constitutes but an index to the gen- 

 eral rise in resisting power accompanying improvement in strength and 

 vitality. In some cases this may be due perhaps to an actual strengthen- 

 ing of local tissues, as in the case of the adult vaginal mucosa, which is 



