304 INFECTION AND RESISTANCE 



stand on the clot. This observation, too, has been rendered incon- 

 clusive by contrary reports from other investigators. Longcope, 29 

 further, has found that alexin was more plentiful in the blood of 

 individuals suffering from leukemia in which of course a larger 

 percentage of leukocytes is present in the circulation. This, too, 

 has been contradicted by other workers, but even if upheld would not 

 influence the possibility of there being alexin in the normal circula- 

 tion. On the whole Gengou's contentions with their consequent 

 bearing upon MetchnikoiFs theory cannot be accepted as final. In 

 fact, the greater part of available experimental evidence seems to 

 point to the actual presence of alexin in the normal circulating blood. 

 This seems also indicated by the unquestionable fact that active 

 phagocytosis may take place in the circulation of an animal and, as 

 we shall see below, free alexin is probably necessary (as opsonin) in 

 this process. Further evidence in this direction also is furnished by 

 the immediate anaphylactic shock which follows the injection of 

 antigen into the blood stream of a sensitized animal, a process in 

 which we have much reason to believe that alexin takes an active 

 part. However, the problem is a difficult one, and, while we 

 favor the opinion that free alexin is present in the intravascular 

 blood, we must admit that a crucial experiment has not yet been 

 formulated. 



Now, as regards the apparent extraction of alexin from leuko- 

 cytes and lymphatic organs, we have already called attention to the 

 fact that most of the researches associating these cells with the bac- 

 tericidal substances were carried out before the dual mechanism of 

 sensitizer and alexin in bacteriolysis had been fully worked out. In 

 consequence conclusions were formulated from the mere facts of the 

 presence of bactericidal or homolytic properties in cell-extracts with- 

 out the further determination of heat stability or the possibility of 

 reactivation. Most of the earlier work also w T as done without suffi- 

 cient attention to the separation of the cells and the serum of leuko- 

 cytic exudates. The first one to do this carefully was Hahn, 30 who, 

 like his predecessors, concluded that the bactericidal leukocytic sub- 

 stances, undoubtedly encountered by him, were identical with alexin. 

 Doubt was first cast upon this by Schatteiifroh, 31 who worked with 

 leukocytes suspended and extracted in physiological salt solution. 

 He found that bactericidal substances were, indeed, obtained in this 

 way, but that, unlike alexin, these substances were thermostable, 

 withstanding exposure to a temperature of 56 C. and destroyed only 

 by exposure to temperatures as high as 75 C. to 80 C. for thirty 

 minutes. 



29 Longcope. Jour, of Hyg., Vol. 3. 



30 Hahn. Arch. f. Hyg., Vol. 25. 



31 Schattenfroh. Arch. f. Ilyg., Vols, 31 and 35, 1897. 



