IMMUNITY AND SUSCEPTIBILITY. 545 



individual to an infection may be dependent upon several things. Our 

 discussion resolves itself under several heads. 



The Protection Afforded the Body by the Surfaces. The body surfaces 

 may be conveniently divided into those which are external and those 

 which are internal. 



Skin and Cutaneous Orifices. The first protective mechanism that 

 we wish to call attention to is the skin. Virulent bacteria are frequently 

 present on the skin of seemingly normal and healthy individuals. Perhaps 

 the most common of these are the Strept. pyogenes and the M. pyogenes 

 vars. aureus et albus. These microorganisms and others live largely as 

 saprophytes, feeding upon the dead and desquamating epithelial cells. 

 However, when it becomes abraded or wounded, the microorganisms may 

 pass in and produce infection. B. tetani, of tetanus or lock-jaw, frequently 

 passes through the skin by means of deep penetrating wounds. The 

 same is true of some other pathogens. 



In case bacteria are successful in permeating the skin either directly 

 or by means of cutaneous orifices, they are usually able to set up a marked 

 inflammation of these structures and produce necrosis of the epithelium. 

 It is in this way that pustules, boils, carbuncles and various forms of 

 cellulitis are produced. The secretions of the sebaceous glands are not 

 germicidal but are perhaps slightly antiseptic due to the salts which are 

 contained therein. Furthermore, as soon as the serum from the blood 

 is extravasated there may be a slight germicidal action from this on the 

 bacteria infecting the skin. The soluble toxins of bacteria cannot be 

 absorbed through the unbroken skin. 



The Subcutaneous Tissue. In case the bacteria are successful in 

 permeating the skin and penetrating the subcutaneous connective tissue, 

 we here again find various protective mechanisms showing themselves. 

 This resistance is due to a very rapid production of new connective tissue 

 which serves to limit mechanically the infection. It is due, furthermore, 

 to the germicidal action of the serum, the mechanical and germicidal action 

 of the fibrin and the phagocytic activity of the leucocytes. These various 

 factors will be discussed subsequently in connection with the phenomena 

 and the protective mechanisms of inflammation. 



The Exposed Mucous Membranes of the Body. The exposed mucous 



membranes of the body usually are covered with a variety of bacteria, 



some of which are pathogenic. Their moist condition favors the growth 



of microorganisms, but the mucus which is secreted upon them forms 



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