554 BACTERIOLOGY. 



hours. When these bacteria are now injected they 

 bring about a distinct immunity within twenty-four 

 hours without inducing any of the unfavorable effects 

 seen in the Haif kine vaccination, such as fever, pain, 

 weakness, etc. The immunity induced in this manner 

 rests upon the formation of specific antibodies. The 

 agglutinated bacteria can be preserved for a long time. 

 The immunization is not dangerous, as no complications 

 have been noted. 



PRECIPITINS. The immunization of animals with a 

 variety of substances other than bacteria has served to 

 shed light upon the complex mechanism of immunity. 

 One of the reactions that is noticed as the result of such 

 immunization is the precipitation observed when the 

 serum of the immunized animal is mixed with the sub- 

 stance with which it has been treated. For instance, 

 the serum of an animal that has received repeated injec- 

 tions of blood from an alien species, will cause a pre- 

 cipitate to form when mixed with the serum of the 

 species of animal from which the blood has been 

 derived. These " precipitins," as they are called, are 

 specific in that they form precipitates only with the 

 serum of the species of animal from which the blood has 

 been derived. If precipitates are formed with the serum 

 of other species of animals it is always in very much 

 lower dilutions. 



This precipitin reaction is so characteristic that it is 

 now employed as the most satisfactory test for blood 

 of a particular species, especially in medicolegal cases 

 requiring the differentiation between human blood and 

 that of the domestic animals. 



In like manner, the repeated injection of milk of one 

 species of animal into another will result in the forma- 



