HO W WE DEFEND URSEL VES FROM UR FOES 9 7 



are apparently exceedingly sensitive to the presence or secretions of 

 microorganisms, for they come out of the blood capillaries shortly after 

 the bacteria have invaded the neighboring tissues. Their mode of 

 attack is frontal; they literally fall upon the intruders and, swallowing 

 them bodily, digest them, so rendering them powerless for any* 

 further activity. 



If the bacteria do not prove very poisonous, the phagocytes are not 

 killed; if however, the poison (toxin) of the bacteria is a virulent one, 

 the leucocytes are killed and their dead bodies constitute " pus," as 

 surgeons call it, or "matter" as other people call it. In suitable prep- 

 arations for the microscope it is possible to see large numbers of mi- 

 crobes in a semi-dissolved state inside the white cells. 



One kind of leucocyte paralyzes or kills the microbes without en- 

 gulfing them. 



Of course leucocytes will do their work well or ill according as they 

 are themselves in good or bad health, vigorous or enfeebled. All ex- 

 hilarating conditions tend to invigorate the leucocytes, all depressing 

 conditions to enfeeble them. 



The leucocytes are, then, the second line of defence — the rank and 

 file of the defending army. When once the outermost physical barriers 

 have been penetrated by the enemy, these living agents take up the 

 defence by active, offensive measures. 



The third mode of defence which we possess is the power of our 

 body-cells to manufacture certain chemical substances having the 

 property of neutralizing the poisons of the bacteria which have invaded 

 us. All the body-cells cooperate more or less vigorously in this the most 

 subtle method of dealing with the soluble toxins manufactured by the 

 bacteria now multiplying in the blood and body-fluids of the unwilling 

 host. 



These soluble toxins affect, stimulate, the tissues of the victim, 

 which, being living cells, react, and the expression of their reaction is 

 the outpouring of a chemical something, appropriately called an anti- 

 toxin which, uniting with the bacterial toxin, neutralizes it and pre- 

 vents it exercising its injurious powers. The infected organism thus 

 works out its own chemical salvation by a vital, but no less chemical, 

 response to the poison of the infection. To do this efficiently is to 

 recover, to fail to do so is to remain infected, to be injured chemically, 

 possibly to die. 



This production of antitoxin on the part of the infected body is a 

 vital, protective mechanism of a chemical order; it is the chemical 

 reply to a chemical insult. If the attacked body-cells can provide 

 sufficient of this antitoxin to neutralize all the toxin made by the 

 bacteria, the individual will not merely get well, but will remain im- 

 mune from that particular infection for a long time, because, when once 



VOL. LXXXV. — 7. 



