116 IMMUNITY AND INFECTION 



consisting collectively of the skin, mucous membranes of the respira- 

 tory and gastro-intestinal tracts and other intact barriers discussed 

 in the preceding chapter. If this initial line of defense holds, the host 

 overcomes the prospective invader and the latter frequently perishes. 

 Repeated microbic assaults may be successful if the first fails. On 

 the other hand, if the microbe prevails and penetrates the initial line 

 of defense, invasion of the tissues of the host occurs and the micro- 

 organism encounters a second line of defense which is made up of 

 two rather distinct factors cellular and humoral. The cellular 

 defense of the host resides in the leukocytes which circulate in the 

 the body fluids and in certain fixed tissue cells in the lungs, lymph- 

 spaces and glands, the Kupfer cells of the liver, as well as large cells 

 which appear in serous cavities. These cells engulf and destroy 

 certain types of invading microorganisms. The humoral defense 

 resides in the natural, non-specific power of the blood and lymph to 

 destroy limited numbers of microorganisms or to so interfere with 

 their nutrition or other functions as to prevent their development 

 within the body. The humoral defense is frequently effective against 

 bacteria which do not succumb to the cellular defense of the body, 

 and vice versa. 



It is recognized that certain environmental factors predispose to 

 infection. Thus, extreme climates, excessive humidity, or exposure 

 to unhygienic conditions, bad air, poor or insufficient food, lack of 

 exercise or fatigue may react upon the individual in ways not defi- 

 nitely understood and reduce his resistance to microbic invasion on 

 the one hand, and his ability to rally his specific, anti-microbic 

 mechanism on the other hand. Intracurrent disease frequently 

 weakens the initial lines of defense, permitting bacteria of the " oppor- 

 tunist" type to become invasive. Thus, furunculosis frequently is 

 a complication of diabetes, pneumonia not uncommonly terminates 

 a case of tuberculosis. Renal and cardiac disease may weaken the 

 normal barriers of the body, permitting a variety of infections with 

 endogenous bacteria. 



It is a well-attested fact that certain occupations or professions 

 cause or promote pathological conditions which predispose to infec- 

 tion. Prominent among these is participation in arts or industries 

 which involve exposure to poisonous or irritating dust or fumes. The 

 incidence of tuberculosis among those frequently exposed to organic 

 or inorganic dust is a striking example of the relation of occupation 

 to infection. 



