INFECTION AND IMMUNITY. 555 



tion of specific precipitins in the blood serum of the 

 treated animal, that will precipitate the milk of that 

 species of animal from which the milk was derived. 



AGGLUTININS. Tn~acquired immunity as the result 

 of recovery from a bacterial disease, and in induced 

 immunity after repeated injections of dead or living 

 cultures of bacteria, the blood serum acquires the prop- 

 erty of agglutinating the bacteria causing the infection. 

 This agglutination results from the action of an antibody 

 in the blood that has the property of causing the clump- 

 ing of the bacteria and the cessation of motility in sus- 

 pensions of motile bacteria. This form of antibody is 

 spoken of as " agglutinin." The specific action of the 

 agglutinin is of such a nature that it has for a long time 

 been employed as a means of diagnosing certain dis- 

 eases, especially typhoid fever, the reaction being known 

 under the name of Widal reaction. 



It has been found that normal blood serum of both 

 man and the domestic animals contains normally agglu- 

 tinins for a variety of bacteria (Bergey). 1 The normal 

 agglutinins are usually present in relatively low amounts, 

 though occasionally individuals are encountered that 

 possess this property to an unusual degree. These 

 "common" agglutinins, as they are called, may be 

 removed from the serum by saturation with related 

 organisms, but the specific agglutinins resulting from 

 immunization with a particular organism are not removed 

 in this manner. 



The agglutination reaction is frequently employed for 

 the purpose of ascertaining the identity of bacteria, and 

 in applying this diagnostic test for this purpose it is 



1 Bergey : Journal of Medical Research, 1908, vol. v., p. 21. 



