IMMUNITY 71 



teria moving quickly through the field, one sees merely 

 several groups of absolutely immobile bacilli. If the 

 reaction is feeble, the clumps are small, and one finds 

 comparatively many isolated and perhaps also moving 

 bacteria. This phenomenon is spoken of as agglutina- 

 tion, and the substance in the serum which brings it 

 about as agglutinin. The clumping thus brought about 

 does not kill the bacteria; moreover, it makes no differ- 

 ence whether the serum is freshly drawn or has been 

 kept for some time, it will agglutinate eciually well, and 

 does not require the addition of fresh serum as do the 

 bacteriolysins. 



Like the antitoxins and the bacteriol3^sin, the ag- 

 glutinins are strictly specific, i. e., a serum from an 

 animal injected previously with typhoid bacilli will 

 agglutinate only typhoid bacilli; one from an animal 

 injected with cholera bacilli will agglutinate only cholera 

 bacilli, etc. 



Since the agglutinins do not kill the bacteria, it may 

 be asked what their function is. Up to the present time 

 we do not know. Through the studies of Gruber and of 

 Widal, however, the agglutinins have come to play a 

 prominent [)art in the diagnosis of bacterial infections, 

 and, in what is called the Widal reaction, afford an im- 

 portant aid in diagnosing typhoid fever. The Widal test 

 in typhoid fever may be performed with blood-serum 

 from the patient or, still simpler, with a drop of ]:)lood 

 collected on a glass slide and allowed to dry. If the 

 latter method is employed, the drop is soaked off with 

 suflficient distilled water to make approximately a dilu- 

 tion containing 1 part of blood in 20 of the mixture. 

 Next, a loopful of this mixture is. mixed with a loopful 



