70 INFECTION AND IMMUNITY 



chemists as proteids. They are sometimes spoken 

 of as " defensive proteids," because they appear to 

 serve as a provision of nature for defence against 

 disease germs. Possibly the increased susceptibility 

 to infection resulting from starvation, great fatigue, 

 and other devitalising agencies is due to a diminution 

 in the quantity of these defensive proteids present in 

 the blood. These germicidal substances differ from 

 the " antitoxins," of which I shall speak in a subse- 

 quent chapter, in the fact that their power to destroy 

 pathogenic bacteria is destroyed by a comparatively 

 low temperature (140 Fahr.). Independent re- 

 searches made by several different investigators seem 

 to show that the defensive proteids of the blood have 

 their origin in the leucocytes, or white blood-cor- 

 puscles, and that an alkaline condition of the blood is 

 favourable, if not essential, to the formation of such 

 germicidal substances, or at least to their release from 

 the leucocytes. The number of leucocytes increases 

 in certain infectious diseases, and this increase, 

 together with an increased alkalinity of the blood, 

 which has been noted, may be a provision of 

 nature for overcoming infection when it has already 

 occurred. 



A more direct role has been ascribed to the leu- 

 cocytes as defenders of the living body against 

 invasion by pathogenic bacteria. 



