230 INFECTION AND IMMUNITY. 



usually is agglutinated with ease. In some in- 

 stances, the typhoid bacillus when freshly culti- 

 vated from a patient, or, indeed, from contami- 

 nated water, has been found to resist agglutination 

 by a strong serum; the same organism after a 

 period of existence on artificial media becomes ag- 

 glutinable. Widal and Sicard noted that often the 

 serum of a typhoid patient would not agglutinate 

 the bacillus which had been cultivated from the 

 patient's own body, although the same serum 

 would agglutinate laboratory cultures. Cultiva- 

 tion of the typhoid bacillus at 42 C. will cause it 

 to lose its agglutinable property, but it may be re- 

 established by subsequent cultivation at lower 

 temperatures. It seems that this variation must 

 be due to some change in the bacteria, i. e., in the 

 agglutinable substance. It is possible that the 

 organism, during its existence in the animal, be- 

 comes immunized against the action of the agglu- 

 tinin just as the animal becomes immunized 

 against the toxic action of bacteria. This condi- 

 tion in the micro-organisms would then be repre- 

 sented by a great excess of agglutinogenic recep- 

 tors, so that a much greater amount of agglutinin 

 would be required to cause clumping. It is read- 

 ily seen how the use of an inagglutinable strain of 

 the typhoid bacillus would affect serum diagnosis. 

 Theories of We are to consider that in the phenomenon of 

 u tion~. agglutination a reaction of a chemical or physico- 

 chemical nature takes place between the agglutinin 

 of the serum and the agglutinogen of the micro- 

 organisms, the actual clumping following as a 

 consequence of this reaction. It is not a "vital" 

 reaction, for dead bacteria may be agglutinated. 



