264 MEDICAL BACTERIOLOGY 



entered the body through the skin or mucous membrane and in a physiologically 

 perfect state, the skin and mucous membranes are impervious to these organisms. 



The blood contains fluids and solids that attack and attempt to remove or 

 destroy any bacteria they meet. Most, if not all, the tissue cells of the body 

 in a physiologically normal state to some degree resist invasion by bacteria. 



In addition to these natural, general, immunizing forces, man acquires 

 other, often stronger, specific resistance forces that act upon a single species of 

 bacteria and are developed as a result of repeated trivial infections by that 

 species or one acute infection by that species or treatment with a vaccine. 



The effectiveness of both general and specific immunizing forces fluctuates 

 with changes in the body state. It is proportional to the physiological condition 

 and vigor of the body as a whole and to that of its various organs. When any 

 of the body as a whole and to that of its various organs. When any of the body 

 functions are impaired, arrested or destroyed, an immediate decline in resistance 

 to infection occurs. Indigestion, insomnia and constipation all lessen resistance. 

 Fatigue, mental or physical, lessens resistance. Of two extremities otherwise 

 equal, differing in that one has been bruised or crushed the injured extremity 

 is most vulnerable to infection. 



There is so slight a difference between the infectiousness of many bacteria 

 and the corresponding resistance of the body, that frequently, the result of 

 lodgment of pathogenic bacteria upon or in the body, whether they cause 

 disease or are resisted, is determined by the state of the body at that time 

 exhausted or not exhausted, well nourished or poorly nourished, constipated or 

 not. 



Immunity, both natural and acquired, to some organisms is so nearly abso- 

 lute that infection seldom follows exposure to it even when vitality is low and 

 physiological activity and coordination poor. 



MECHANISM OF IMMUNITY 



Metchnikoff has clearly shown that many of the tissue cells and especially 

 endothelial cells and polymorphonuclear leucocytes elaborate a ferment-like 

 substance Cytase which destroys or facilitates the destruction of bacteria. 

 Wright disclosed the presence in blood serum and fluid expressed from muscular 

 and connective tissues of a substance called opsonin which facilitates ingestion 

 and digestion of bacteria by phagocytic cells. Pfeiffer discovered that under 

 certain conditions peritoneal exudate causes a granular degeneration of the 

 cholera vibrio. 



PFEIFFER'S PHENOMENON 



Pfeiffer's test (page 123) is explained, on experimental evidence, by Metchni- 

 koff as follows: 



Immediately after the injection of living cholera vibriones into the peritoneal 

 cavity of a guinea-pig, disintegration of the phagocytic cells surrounding the 

 organisms occurs Phagolysis with a consequent liberation of their cytase. 

 The cytase liberated by the cells of susceptible animals is not able to injure 



