288 Dr. Edward E. Klein [Feb. 20, 



being able to cope with the Bacteria, or being altogetber indifferent 

 to the presence of the enemy ; when this is the case in an animal or 

 human body, the phagocytes being powerless to destroy the Bacteria, 

 we are supposed to be dealing with a body that is susceptible to the 

 disease ; but when the phagocytes do their duty, then the body is 

 unsusceptible to the disease. Again, when an animal or human being, 

 by a mild first attack, or by protective inoculation of one kind or 

 another, becomes unsusceptible to a second attack, this is explained 

 by saying that, though the phagocytes have not done, or have not 

 been able to do, their duty during that first attack, they have now 

 been rendered capable of doing it. 



Now, if you ask what is the evidence on which this theory of 

 phagocytosis is based, you will find that it is of the most slender kind, 

 and you will further find that there is an overwhelming number of 

 observations which directly negatives this theory of universal phago- 

 cytosis, and, moreover, proves conclusively that if phagocytosis has 

 any share in producing a refractory condition on the part of animals 

 towards a particular infectious disease — be that a primary unsuscepti- 

 bility or an acquired immunity against a second attack — this share is 

 of a remarkably small degree. The whole theory was started by 

 Metschnikoft' by the interesting and fundamental observation that, if 

 anthrax bacilli are introduced into the dorsal lymph-sac of the frog — 

 an animal unsusceptible to anthrax while living under normal con- 

 ditions — the bacilli become inclosed in the lymph-cells, and are 

 gradually broken up ; they do not multiply, and do not therefore set 

 up the disease anthrax. This observation, which is easily verified, 

 was the starting-point for the theory. Metschnikoff and others have 

 described similar appearances in other conditions of refractory states. 



Now the above observation is explained by Metschnikoff in this 

 way : the lymph-cells are acting the part of guardians, swallowing up 

 the bacilli and preventing them from entering the circulation, and 

 thereby preventing the outbreak of the disease. It must seem very 

 extraordinary that this should be really a true explanation of the 

 refractory state of the frog towards anthrax, considering that the 

 bacilli, like other minute particles, when injected into the lymph-sac, 

 would be absorbed and brought into the circulation in a few minutes, 

 nay, seconds — at any rate some hours before the phagocytes have got 

 into the lymph-sac in sufficient numbers to do battle with the bacilli. 

 That the bacilli really enter the circulation in this and other cases, 

 but are destroyed by the blood, not by the leucocytes, but by 

 the fluid part of the blood — the plasma — has been abundantly proved ; 

 and it has likewise been proved that the fluid part of the blood and 

 of the lymph in general has a remarkable germicidal action, inde- 

 pendent of any cellular element, leucocytes, or other cells. The 

 observation of Metschnikoff admits of an explanation different from 

 that given by him ; it may mean, and probably does mean, that the 

 bacilli cannot exist in the fluid of the lymph and of the blood ; they 

 re destroyed here, but they tahe refuge in the leucocytes or lymph-cells 



