140 THEORIES CONCERNING IMMUNITY 



When they are very numerous and able to obstruct the cir- 

 culation more or less completely, the animal soon succumbs; 

 but it is conceivable that in certain cases, they may deter- 

 mine local lesions of small size and the animals may survive." 



Other similar experiments convinced this savant that the 

 injection of blood from cattle into dogs represents only one 

 special case of this kind of "coagulating injection/' so that 

 one may generalize this fact and say: 



The second injection of blood or of a heterologous albumi- 

 noid substance (from an animal of a different kind) into the 

 veins of an animal several days after the first injection, is 

 much more dangerous than the first. 



The first injection of a strong dose is usually well endured, 

 the second and lesser dose is usually fatal. 



The second dose is pathogenic because it causes the forma- 

 tion of a precipitate which obstructs the capillaries. 



Hayem pointed out this phenomenon without endeavoring 

 to explain it, so that his discovery did not awaken at that 

 time the interest which it deserved. 



It was only several years later (1894-1900) that all its 

 importance was realized through the works of Pfeiffer, 

 Metchnikoff, J. Bordet, Kraus, Belfanti and Carbone, and 

 others, on the reciprocal reactions between the sera of 

 immunized animals, on the one hand, and bacilli, the broth 

 cultures, sera, blood elements, or other injected cells, on the 

 other. Thus Pfeiffer determined that cholera vibrios injected 

 into the peritoneal cavity of a guinea-pig, previously immun- 

 ized against these bacilli, were agglutinated and partially 

 destroyed in the peritoneal fluid, without direct intervention 

 of phagocytes. 



Soon after, Metchnikoff, followed by J. Bordet, showed 

 that the same phenomena could be reproduced in vitro by 

 mixing in a test-tube a culture of cholera vibrios with a small 

 amount of serum from an immunized animal. 



Kraus obtained the same result, not only for cholera, 

 typhoid and plague bacteria, but also for the filtrated broth 

 cultures of these microbes. In mixing these broth cultures 

 with a little serum from immunized animals, he saw the 

 formation of a precipitate. 



