UNION OF ANTIBODIES WITH ANTIGENS. 347 



Union of Ag- 

 glntinin and 

 Amboceptor 

 with Cell 



Granting, then, that neutralization of toxin by 

 antitoxin is of a chemical nature, the first essential 

 step in the chemical or side-chain theory is estab- 

 lished. If antitoxin combines chemically with 

 toxin, union must occur through combining groups 

 which each molecule possesses. Herein lies the ex- 

 perimental justification for assuming the existence 

 of haptophorous groups. 



The situation is more difficult in regard to the 

 union of receptors of the second and third orders, 

 i. e., agglutinins and amboceptors with the ho- Receptors. 

 mologous receptors of bacteria and other cells. 

 One can not titrate bacteria against agglutinin or 

 bactericidal amboceptors so exactly as toxin can be 

 titrated against antitoxin, for, in the first place, 

 it is difficult to obtain at will a desired concentra- 

 tion of bacteria and to keep it without alteration, 

 and, in the second place, bacterial cells contain 

 many more receptors than are necessary for their 

 agglutination and solution. A given mass of bac- 

 teria will take up varying quantities of agglutinin, 

 depending on the concentration of the latter, and 

 the same principle applies to the absorption of 

 bactericidal and hemolytic amboceptors. As more 

 and more agglutinin is added, the total amount 

 absorbed increases with each addition, although 

 the ratio of absorbed to unabsorbed agglutinin 

 grows less continuously. The conditions which 

 govern this phenomenon are not understood. 

 Perhaps no condition speaks more decisively for 

 chemical union of these bodies with cell receptors 

 than immunization experiments which were car- 

 ried on with cells which had been treated with a 

 great excess of the specific antiserum. The as- 



