94 THE PRINCIPLES OF IMMUNOLOGY 



bacteria and other cells are subject to agglutination, but no definite 

 proof is at hand to show that the ectoplasmic substance is not of 

 considerable importance. The influence of salts on agglutination 

 lends much support to the conception that agglutination is a col- 

 loidal phenomenon. As has been indicated above, the presence of 

 electrolytes is essential to the reaction, but salts, acids, and salts of heavy 

 metals, if present in sufficient concentration, may of themselves produce 

 agglutination. On the other hand, salts in strong concentration 

 serve to prevent the action of agglutinin. When bacteria have ab- 

 sorbed agglutinin, very small amounts of salt serve to bring about 

 agglutination. If a suspension of bacteria and an agglutinating 

 serum are each dialyzed free of salt and the two mixed, the bacteria 

 absorb agglutinin. This is shown by the fact that the supernatant 

 fluid after centrifugalization is free of agglutinin, but agglutination 

 occurs on addition of salt. Bordet interpreted the phenomenon of 

 agglutination as having two phases, first that of sensitization of the 

 bacteria by the agglutinin, and second, that of agglutination of these 

 agglutinin-bacteria by the salt. It may be stated in other terms 

 that the bacteria are primarily suspensions of protected colloids 

 which are so altered by the agglutinin that they become unprotected 

 and precipitable by salts, or that they become more permeable for 

 electrolytes. In fact, it has been shown that sensitized bacteria 

 take up salts more readily than unsensitized. The similarity of 

 bacteria to protected colloids is also borne out by Porges, who 

 showed that while encapsulated organisms are inagglutinable, the 

 solution of their capsules by heating in weak acid renders the bac- 

 teria agglutinable. Bacteria carry electro-negative charges and move 

 toward the anode, whereas agglutinins are electro-positive. The 

 sensitized bacteria are agglutinated by the current between the poles, 

 although the sensitized bacteria move slowly toward the anode. 

 The small amount of salt necessary for agglutination further sup- 

 ports the influence of electrical charge and thus furnishes further 

 analogy with colloidal precipitation. Neisser and Friedemann have 

 studied the similarities of agglutination and colloidal precipita- 

 tion and offer much in support of such analogy. Two protocols 

 may serve to show the importance of their work, one dealing 

 with the so-called sensitization and the other with inhibition zones. 

 Just as salt influences agglutinin and agglutinogen, so may it 

 influence mastic and gelatin solutions, as may be seen in the follow- 

 ing experiment : 



i.o c.c. mastic i.o c.c. mastic + o.oooi c.c. 

 (i-io original emulsion) 2% gelatin sol. and 



10% NaCl Sol. diluted to 3.0 c.c. diluted to 3.0 c.c. 



I.O C.C. + + 



0.5 c.c. + . 



0.25 c.c. 



0.125 c.c. + 



0.05 c-c. 



0.025 c.c. 



