130 BACTERIOLOGY FOR NURSES 



are produced without attributing the action to any one group of 

 cells. 



Recent experiments tend to show that the leukocytes, the spleen, 

 lymphatic tissue, and bone marrow are all actively concerned in 

 manufacturing antibodies. In infections due to the various patho- 

 genic cocci, phagocytes appear to be the most active agents ; in 

 other infections such as those due to typhoid and other bacilli 

 it is probable that antibodies are chiefly operative in overcoming 

 the infection and producing immunity. Thus there appears no 

 warrant for holding one view to the absolute exclusion of the other. 

 It would seem that all phases of immunity are cellular in origin 

 and that this cellular activity is general rather than limited to 

 one group. 



Phagocytosis. ' In order to understand phagocytosis it is 

 necessary to consider its three phases ; namely, the advance of the 

 leukocytes, the engulfment of the particles, and their subsequent 

 digestion. The question naturally arises as to how the leukocytes 

 are made aware that their presence is needed at the point of infec- 

 tion ; some attractive force must operate between this point and 

 the leukocytes circulating in the blood. It is assumed that a chem- 

 ical substance acts as an attractive force. Almost all motile 

 unicellular organisms, whether animal or vegetable, will respond 

 to chemical stimuli. Generally attraction is toward a given point, 

 constituting what is known as positive chemotaxis. 



Experiments show that such cell movement is due to a change 

 in surface tension. If the chemical substances which cause this 

 apparent stimulus decrease the surface tension the cell advances 

 in the direction from which the substance comes, if it decreases 

 the tension the cell recedes. 



" This explanation may account for the behavior of cells in 

 inflammation. At the point of injury or infection chemical sub- 

 stances are produced that tend to lower the surface tension; 

 these are carried by the body fluids to the nearest capillaries 

 where they enter through the vessel wall and come in contact with 

 the leukocytes. Naturally the surface tension will be least on 

 that side of the vessel wall through which the stimuli has passed 



