518 THE RELATION OF COLLOIDS AND LIPOIDS TO IMMUNITY 



agglutination or to give but a slight reaction. This phenomenon has 

 been explained, according to Ehrlich's side-chain theory, as due to the 

 presence of agglutinoids that have a great affinity for the bacteria and 

 unite with them without being active in the free state, owing to a loss 

 of the agglutinophore portion of the molecule. Each cell united with 

 an agglutinoid is one cell less to undergo agglutination by agglutinin, 

 and accordingly in weak dilutions of serum agglutination is feeble or 

 absent whereas in higher dilutions the phenomenon may be clearly 

 observed. 



Other explanations of the action of agglutinins and precipitins, 

 based upon colloidal reactions, have been advanced. Thus Neisser and 

 Friedmann have shown that suspensions of mastic may be "protected" 

 against the precipitating action of ferric hydroxid by the addition of a 

 small amount of organic colloid, such as serum, leech extract, or extract 

 of typhoid bacilli, regardless of whether this colloid is charged posi- 

 tively or negatively or is neutral. The aforenamed observers believe 

 that normal bacteria may be surrounded by a similar protective envelop 

 that prevents the agglutinating action of substances of opposite sign. 

 The action of agglutinin, therefore, would be to remove this layer, so 

 that the ions of opposite electric charge can unite with the bacteria and 

 bring about their agglutination. This may be an explanation of the 

 role of salts in the phenomenon of agglutination, the agglutinins remov- 

 ing the protecting envelops and the salt furnishing the ions of opposite 

 charge that bring about agglutination. 



Owing to the fact that a discrepancy arises here for the reason that 

 emulsions of red corpuscles are agglutinated by both positive and 

 negative colloids (ferric hydroxid and cuprum ferrocyanid), Girard, 

 Mangin and Henri have given the following explanation of agglutina- 

 tion: When a red corpuscle is suspended in a fluid, various salts, es- 

 pecially the sulphates of magnesium and calcium, are diffused, which 

 tends to facilitate the precipitation of negative and positive colloids, 

 so that each corpuscle comes to be surrounded by a layer of precipitated 

 colloid material. This zone of precipitated colloids of either negative 

 or positive charge determines agglutination in the presence of a colloid 

 solution of opposite charge, such as agglutinin or inorganic colloids 

 (silicic acid, etc.). 



3. Hemolysins. Reference has been made elsewhere to the original 

 observations of Bordet, showing that red corpuscles may absorb much 

 more hemolytic antibody than is necessary to bring about their lysis, 

 and that this absorption is analogous to colloidal absorption. 



