262 RECEPTORS OF THE SECOND ORDER 



substance within the introduced cell which acts as a stimulus 

 (antigen) to the body cells is called an ''agglutinogen.'' 

 That "agglutinogen" is present in the cell has been shown by 

 injecting animals experimentally with extracts of cells (bac- 

 terial and other cells) and the blood serum of the animal 

 injected showed the presence of agglutinin for the given cell. 

 It will be noticed that the receptors which become the free 

 agglutinins have at least two functions, hence at least two 

 chemical groups. They must combine with the foreign cells 

 and also bring about their clumping together, their aggluti- 

 nation. Hence it can be stated technically that an agglu- 

 tinin possesses a haptophore group and an agglutinating 

 group. 



It is probable that the agglutination, the clumping, is a 

 secondary phenomenon depending on the presence of certain 

 salts and that the agglutinin acts on its antigen as an enzyme, 

 possibly a "sphtting" enzyme. This is analogous to what 

 occurs in the curdling of milk by rennet and in the coagula- 

 tion of blood. This probability is substantiated by the 

 fact that suspensions of bacteria may be "agglutinated" 

 by appropriate strengths of various acids. 



The formation of agglutinin in the body for different bac- 

 teria does not as yet appear to be of any special significance 

 in protecting the animal from the organism, since the bac- 

 teria are not killed, even though they are rendered 

 non-motile, if of the class provided with flagella, and are 

 clumped together. The fact that such bodies are formed, 

 however, is of decided value in the diagnosis of disease, and 

 also in the identification of unknown bacteria. 



In many bacterial diseases, agglutinins for the particular 

 organism are present in the blood serum of the affected 

 animal. Consequently if the blood serum of the animal be 

 mixed with a suspension of the organism supposed to be the 

 cause of the disease and the latter be agglutinated, one is 

 justified in considering it the causative agent, provided 

 certain necessary conditions are fulfilled. In the first place 

 it must be remembered that the blood of normal animals 

 frequently contains agglutinins ("normal agglutinins") 

 for many different bacteria when mixed with them in full 



