RELATION OF LEUKOCYTES TO IMMUNITY 301 



inactivated exudate, however, had lost vitality during the process of 

 isolation and washing, and no longer possessed secretory power. 



Hankin, 12 Kanthack and Hardy 13 had gone even farther than 

 this, and had attributed the production of alexin to the eosinophile 

 leukocytes particularly. 



Metchnikoff, 14 basing his opinion on his own studies, those of 

 his pupils, and many other investigations similar to those mentioned 

 above, came to the conclusion that, under ordinary conditions, the 

 normal blood contains no free bactericidal substances. He assumes 

 that these substances are entirely intracellular, being constituents of 

 the various phagocytic elements, by means of which the cells digest 

 the foreign elements they take up. He believes that there are essen- 

 tially two varieties of such digestive enzymes or "cytases" just as 

 there are two varieties of phagocytes. The microphages, chiefly con- 

 cerned in the digestion of bacteria, secrete the bactericidal alexin, or, 

 as Metchnikoff calls it, "microcytase." The macrophages, the large 

 mononuclear lymph and endothelial cells, primarily concerned in the 

 phagocytosis of cellular elements (red cells, etc.), contain another 

 variety of digestive enzyme, the "macrocytase," or cytolytic (hemo- 

 lytic) alexin. The supposition that the hemolytic "cytase" is largely 

 derived from the macrophages was based particularly upon the in- 

 vestigations of MetchnikofFs pupil, Tarassewitch, 15 who found that 

 the extracts obtained from lymph nodes, and other organs rich in 

 macrophages, possessed hemolytic properties. Both this work and 

 the preceding studies regarding the extraction of alexin from poly- 

 nuclear leukocytes will be more fully discussed below. 



Maintaining that these cytases are purely intracellular under 

 ordinary conditions, Metchnikoif believes that, in normal animals, 

 the destruction of invading bacteria or of injected cellular substances 

 (blood cells, etc.) is accomplished entirely by the phagocytic process, 

 with subsequent intracellular digestion. In immunized animals, 

 however, there is present in the circulating blood another substance, 

 not identical with the cytases, but also derived from the leukocytes or 

 from the blood-forming organs the a fixateur" (Ehrlich's "ambo- 

 ceptor" Borders "sensitizer"). This specific "fixateur" sensitizes 

 the bacteria or other antigens to the action of the cytases. For his 

 assumption regarding the origin of this sensitizer he finds support 

 largely in the researches of Pfeiffer and Marx, and others mentioned 

 in our section on the origin of antibodies, as well as in the simi- 

 lar investigations of Deutsch, 16 carried on under MetchnikofFs per- 

 sonal supervision. 



12 Hankin. Centralbl f. Bakt., Vol. 12, 1892. 



13 Kanthack and Hardy. Proc. Eoy. Soc., Vol. 52, 1892. 



14 Metchnikoff. Ann. de VInst. Pasteur, Vol. 7, 1893; Vol. 8, 1894; Vol. 

 9, 1895. 



15 Tarassewitch. Ann. de VInst. Past., Vol. 16, 1902. 



16 Deutsch. Ann. de VInst. Pasteur, Vol. 13, 1899. 



