308 THE HUMAN BODY 



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by chemical action. To this substance is given the name 

 It has been shown to be made up of two other substances. The 

 first of these, the complements, are present in the blood in variable, 

 but considerable, amounts and are actively destructive agents. 

 Their destructive power is limited, however, by the circumstance 

 that they are unable to attack foreign organisms directly, but must 

 first be in combination with the second bodies, known as inter- 

 mediary or immune bodies, through which they gain the necessary 

 chemical grasp on the cells which are attacked. An important 

 feature of the immune bodies is that each kind can combine with 

 only one sort of foreign cell. Unless immune bodies of the right 

 kind are present, the complements are helpless. The analogy of 

 the yale lock which can be opened only by its own key suggests 

 itself. Clearly the scope of this protective mechanism is limited to 

 the varieties of immune bodies that happen to be present. 



Why Does Infection Ever Occur? The establishment of an 

 infection in the face of this elaborate protective mechanism can be 

 explained in one of two ways. Either the mechanism falls off in 

 efficiency, which is the condition present when we say "the re- 

 sistance of the Body is lowered," or the invading organisms are so 

 virulent that the Body is unable to overcome them. Lowered 

 body resistance may result from a number of conditions; under- 

 nutrition, prolonged exposure to extremes of heat or cold, alco- 

 holism, severe local inflammations, all of these may diminish the 

 number of phagocytes or the quantity of alexins, or may lessen 

 their activity. Bacteria may vary from time to time in virulence. 

 It appears that the virulence of most sorts is much increased by a 

 period of growth in a living Body. It is because of this increase 

 of virulence that "exposure" to an infected individual is so often 

 followed by infection. The fact of increased, virulence explains 

 also the occurrence of "epidemics." 



Recovery from Infection involves two processes: 1. Destroying 

 and getting rid of the enormous numbers of bacteria which de- 

 velop during the progress of the disease; 2. Getting rid of or neu- 

 tralizing the poison, or toxin, which the bacteria produce and which 

 is the real cause of trouble. The course of every infection is a 

 struggle between the Body on one hand and the micro-organisms 

 on the other. The outcome is recovery or death according as one 

 side or the other proves victorious. 



