FUNCTIONS OF THE BLOOD 275 



do establish themselves flourish mightily, at least for a time. 

 It follows, therefore, that bacteria are ordinarily forcibly pre- 

 vented from establishing themselves. This prevention of infection 

 is one of the functions of the blood. It must be sharply differen- 

 tiated from an additional disease-resisting function possessed also 

 by the blood, which is the overcoming of infection after it is once 

 established. In the absence of this latter function every infec- 

 tion would result fatally. 



The Infection-Resisting Mechanism. Two sorts of structures 

 in the blood are engaged in the destruction of invading micro- 

 organisms. They work independently but in cooperation. The 

 first of these are the phagocytes previously mentioned (p. 266) 

 which engulf and thus dispose of the invading foreign bodies. 

 The second sort is not made up of formed elements like the phago- 

 cytes, but is in solution in the plasma. It attacks and destroys 

 bacteria by what seems to be an enzym action. To this sub- 

 stance is given the name alexin. It has been shown to be made 

 up of two other substances, the complements, which are probably 

 formed as part of the leucocytes and given off to the blood when 

 they disintegrate, and the intermediary bodies which are constant 

 constituents of the plasma. The complements are thought to be 

 the actively destructive agents, but to be unable to attack foreign 

 cells except when in combination with the intermediary bodies. 

 Since the complements are derived from disintegrating leucocytes 

 their number is variable, depending on whether the blood is rich 

 or poor in these structures. 



Why Does Infection Ever Occur? The establishment of an 

 infection in the face of this double 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 



