THE SIDE CHAIN THEORY 107 



bacterial emulsion (e. </., 1 c.c.) are treated with an increasing number 

 of agglutinin units it will be observed that the bacteria have the power 

 of absorbing an enormous excess of agglutinin beyond the amount 

 that is actually required to produce agglutination. If, moreover, 

 the number of units that is actually absorbed is compared with the 

 number added, the interesting fact develops that with increasing 

 concentration of the agglutinins the absolute absorption by the bacteria 

 rises, while the absorption coefficient, i. e., the ratio between the 

 number of units added and the amount absorbed, falls. This is 

 well shown in the accompanying table which is taken from Eisenberg 

 and Volk. 



ANTITYPHOID SERUM, ZOROASTER III. AGGLUTINATION VALUE = 45,000 UNITS; 



Agglutinin units Agglutinin units Coefficient of 

 Serum dilution. added. absorbed. absorption. 



to 20000 2 2 1.0 



to 2000 22 22 1.0 



to 1000 45 45 1.0 



to 600 75 75 1.0 



to 500 90 89 0.99 



to 200 225 210 0.93 



to 100 450 400 0.88 



to 20 2250 1650 0.73 



to 4 11250 6750 0.60 



to 2 ..... 22500 12500 0.56 



to 1 45000 22500 0.50 



The question then arises how to explain the apparent paradox 

 that the same quantity of bacteria which can only absorb 12,500 

 units out of 22,500 that have been offered, can actually absorb 

 22,500 when brought in contact with a proportionately large amount. 

 Upon first consideration the thought of a chemical union between 

 agglutinin and agglutinable substance would seem to be out of the 

 question. Various explanations, however, have been offered, any one 

 of which would show that the paradox is in reality only apparent. 

 As will be seen later on, there are reasons for supposing that an 

 agglutinating serum may contain not only one single agglutinin, but 

 a number of agglutinins which correspond to the presence of an 

 equal number of agglutinable substances (agglutinogens) in the body 

 of the bacillus; as experience, moreover, has shown that different 

 antigens, even though closely related, may differ very considerably in 

 their antibody forming power, we may assume that the number of 



