550 THE RELATION OF COLLOIDS AND LIPOIDS TO IMMUNITY 



the cathode, and, as we have seen, for the neutralization and precipita- 

 tion of colloids the solution of colloids should be of opposite sign. It 

 must be remembered, however, that toxins and antitoxins react in very 

 complex fluids containing other substances consisting of both colloids 

 and electrolytes, and until the electric charge of these in pure form is 

 determined, the apparent similar electric charge of toxin and antitoxin 

 can hardly outweigh the otherwise remarkable analogy it bears to col- 

 loidal reactions. 



2. Agglutinins and Precipitins. Various theories in explanation of 

 the phenomenon of agglutination have been described in a previous 

 chapter. The theory of Bordet appears to be best, and is based upon 

 certain principles of colloidal chemistry. When bacteria are suspended 

 in a fluid free from salt, agglutination does not take place because the 

 bacteria carry a similar negative charge of electricity. When, however, 

 ions of positive charge are added, as, e. g., sodium chlorid, the bacteria or 

 other cells are repelled and coalesce to form masses, according to the 

 laws of surface tension, in an effort to protect themselves. Larger 

 masses may be formed that finally come within the influence of gravity 

 and are deposited at the bottom of the test-tube. According to the 

 same laws, the addition of agglutinin removes the negative charge of 

 bacteria or other cells, with the consequent formation of clumps and 

 masses. Similar phenomena may be observed in the precipitation of 

 colloidal suspensions of clay in distilled water by the addition of a salt. 



Solutions of inorganic colloids, as, for example, that of silicic acid, 

 may agglutinate red corpuscles; bacteria, such as suspensions of typhoid 

 and colon bacilli, may be agglutinated by solutions of the ferric salts. 



Just as an excess of one colloid solution will charge masses of the 

 other, resulting in a repelling action and breaking up of the agglutinated 

 clumps, so the addition of an excess of agglutinin is found to prevent 

 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 hi 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 



