80 THE BACTERICIDAL SUBSTANCES OF THE BLOOD 



plement has not been proved, but we are also forced to admit that 

 no other cells have as yet been shown to form complement. As long 

 as the latter was regarded as a single substance this negative search 

 might very naturally lead one to think that our technique may have 

 been imperfect, but now, when we know that what we call comple- 

 ment is very evidently composed of two constituents, which can be 

 separated from one another and then reunited to reform active com- 

 plement, the possibility suggests itself that these two components 

 may have a separate origin, and it will accordingly be necessary to 

 repeat the search from this standpoint. 



Leukins. Since the leukocytes have been virtually eliminated as 

 the source of the serum complement, in the older sense of the word, 

 while their bactericidal action toward certain organisms at least is 

 an established fact, we are forced to the conclusion that the body has 

 at its disposal still other defensive factors than those with which we 

 have thus far become acquainted. To what extent such substances 

 occur in the tissues at large still remains to be determined. A priori 

 it would seem reasonable to expect that they might be present in all 

 cells, but thus far their production by the leukocytes only has been 

 satisfactorily established. 



These leukocytic alexins, as we may term them, using the word 

 alexin in the original sense, viz., synonymously with "protective 

 substances," have been variously described as leukins and leukocytic 

 endolysins by Schneider and Peterson respectively, and the former 

 seems to have proved quite conclusively that these bactericidal sub- 

 stances are actually secreted by the leukocytes, as Buchner originally 

 claimed for the common alexins of the serum. This was demon- 

 strated by placing leukocytes for 30 minutes in diluted blood-serum 

 (5 per cent, in normal salt solution), when it could be shown that the 

 solution had developed very active bactericidal properties. During 

 this process the leukocytes were not destroyed, but could be made 

 to furnish additional, equally active extracts without impairment of 

 their vital functions (power of phagocytosis, locomobility). If the 

 same experiments were carried on in an atmosphere of carbon dioxide 

 the solutions developed no bactericidal properties, while a trans- 

 ference of the leukocytes to ordinary conditions again led to an active 

 production of "leukins," the cells having apparently not been 

 damaged by their exposure to the carbon dioxide. Corresponding 

 results were obtained with Bier's congestive lymph, and satisfactory 



