302 INFECTION AND RESISTANCE 



Final digestion of the sensitized antigens (bacteria or blood 

 cells), however, can take place only under the influence of the 

 cytases intracellularly, unless by previous leukocytic injury these 

 enzymes have been liberated into the blood stream. 



While it is admitted, then, that bacteria may be killed and di- 

 gested both intra- and extracellularly in the animal body, the cytases, 

 which accomplish this, are regarded as the same in both cases, being 

 derived from the phagocytic cells. In immunized animals "fixateur" 

 may be produced under the stress of active immunization and fur- 

 nished to the blood stream by the blood-forming organs. By this 

 substance bacteria and cells may be sensitized. However, the enzyme 

 by which digestion is actually accomplished, "cytase" or alexin, is 

 not present free in the blood even in immune animals unless it has 

 become free and extracellular by injury to the leukocytes. 



How, then, on this basis does Metchnikoff account for the Pfeiffer 

 phenomenon, in which the extracellular destruction of bacteria takes 

 place rapidly in the peritoneal exudate ? His explanation is the fol- 

 lowing: When bacteria or other substances are injected into the 

 peritoneum there is a preliminary injury of leukocytes (phagolysis), 

 and by this alexin or cytase is liberated. When cholera spirilla, for 

 instance, are injected into the peritoneal cavity of an immunized 

 guinea pig there follows a short period during which few if any 

 leukocytes are present in the exudate, but many may be found gath- 

 ered in motionless clumps in the folds of the peritoneum and mesen- 

 tery, incapable of phagocytosis and apparently injured. If such 

 leukocytic injury can be avoided Metchnikoff claims that the extra- 

 cellular lysis of cholera spirilla will fail to take place. Thus if 

 sterile broth or salt solution is injected into the peritoneum of a 

 guinea pig a preliminary phagolysis will be followed by an accumu- 

 lation of leukocytes. If cholera spirilla are now introduced no 

 extracellular digestion is seen, but, instead of this, rapid phagocytosis 

 takes place. This he says is due to the fact that the freshly accumu- 

 lated, healthy phagocytes, collected in response to the preliminary 

 broth injection, are not easily injured and do not undergo phago- 

 lysis; no cytase is liberated and, in consequence, no serum bacterio- 

 lysis can take place. In the same way he claims that the hemolysis 

 of red blood cells (goose blood) in the peritoneum of specifically 

 immunized guinea pigs may be prevented if, by a previous injection 

 of broth, healthy leukocytes have been caused to accumulate. In 

 such a case the goose blood corpuscles are rapidly ingested by the 

 phagocytes and no hemolysis occurs. 



It is self-evident that the validity of this interpretation of the 

 occurrences is strictly dependent upon the demonstration that the 

 circulating blood normally contains no alexin or complement. This 

 is rigidly maintained by Metchnikoff, and is indeed one of the most 

 important uncertainties of serology. He asserts that alexin appears 



