INFECTION AND IMMUNITY. 577 



teria are now injected they bring about a distinct immu- 

 nity within twenty-four hours without inducing any of 

 the unfavorable effects seen in the Haffkine 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 intimate mechanism of immunity. 

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

 immunization is the formation of precipitous 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. Albuminous urine or 

 other albuminous fluids may also be employed in the 

 immunization of the animals arid similar agglutinins are 

 formed which produce precipitates in the blood serum, 

 37 



