CHAPTEE IX. 



NATURAL IMMUNITY. 



Natural immunity to infection depends, first, on 

 certain obstacles to invasion which are afforded 

 by the body surfaces and the germicidal effect of 

 their secretions; and, second, on antibacterial and 

 antitoxic forces which are present in the cells and 

 fluids of the interior body. 



(1) Protection Afforded by the Body Surfaces 



Virulent organisms (e. g., staphylococci and The skin. 

 streptococci) exist normally on the skin or be- 

 tween the superficial horny cells, some exceptional 

 circumstance being necessary, e. g., wounds, to en- 

 able them to penetrate deeper and to cause disease. 

 It is evident, then, that the physiologic shedding 

 of the superficial horny cells and their continual 

 reformation at a deeper level is a process calcu- 

 lated to rid the surface of the body of many micro- 

 organisms. 



The question whether micro-organisms can ever 

 penetrate the unbroken skin has been much dis- 

 cussed. Although experiments have shown that 

 traumatism is not absolutely necessary, clinical 

 experience indicates that these so-called crypto- 

 genetic infections are not of ready occurrence. 

 When they do occur, the infection atrium is prob- 

 ably one of the glandular orifices. 



The sweat glands with their ducts, and the hair cutaneous 

 follicles with their appended sebaceous glands, 



