GERMICIDAL SERA 219 



isms of symptomatic anthrax provided all goes into the blood current 

 directly and the subcutaneous tissue is not infected. 



Immune Sera. The words, immune sera, are likely to be mis- 

 leading in this connection. Immunity due to bactericidal constitu- 

 ents of the blood, whether it be natural or acquired, is always rela- 

 tive. Even the immunity secured by one attack of the disease may be 

 overcome, in most instances at least, by the administration of an over- 

 whelming dose of the virus in virulent form. The man who has 

 once had smallpox may have it again when brought into intimate 

 and long-continued contact with the virus. The immunity induced 

 by one attack of yellow fever is probably the most complete and per- 

 sistent form known among the diseases to which man is susceptible. 

 The only way in which a man who has had this disease may be exposed 

 again is through the bite of an infected mosquito, and the virus intro- 

 duced in this way is always limited and fairly constant in amount. 

 We do not know what would happen to a yellow fever immune should 

 100 or 1,000 infected mosquitos bite him in the same hour. Experi- 

 mentally the injection of a large number of bacteria into the peritoneal 

 cavity of highly immune animals kills quickly, and the more marked 

 the immunity, provided it be due to germicidal substances in the blood 

 and lymph, the more promptly and more certainly does it kill. The 

 explanation of this fact is that the fluids of the body promptly split 

 up the bacterial cells, setting the protein poison free in large amount. 

 Nothing exactly similar to this occurs in nature, but the experiment 

 is of great value in demonstrating the phenomena of bacteriolysis. 

 A highly germicidal blood is of great value in preventing infection, 

 because the first few organisms that find their way into the body are 

 promptly killed before they can multiply and while the amount of 

 poison set free is too small to produce any marked effect. On the 

 other hand, it would be highly disastrous for the blood to become sud- 

 denly and highly germicidal when the body is filled with bacteria. This 

 would mean sure and speedy death. Phagocytes not only serve the 

 body in feeding on bacteria, but they protect the body from the poison 

 which results from the extracellular cleavage of the bacteria. 



The essential difference between the germicidal constituent of nor- 

 mal serum and that of immune serum is that the latter is specific 



