442 BACTERIOLOGICAL CHEMISTRY 



hapten, following which, as a rule, the secondary reactions 

 do not take place. There are certain haptens, however, 

 notabl}^ the soluble specific substances and polyvalent 

 haptens, sometimes called complex haptens, which 

 lead to precipitin reactions, the products of their reaction 

 with antibodies being insoluble. That reaction between 

 hapten and antibody in the case of simple (non-pre- 

 cipitating) haptens also occurs is shown by the fact that an 

 antibody so treated will no longer react with the complete 

 antigen, its affinity for the active groups of the antigen 

 having been satisfied by those of the hapten. Moreover it 

 has been show^n by direct means that the hapten combines 

 with the antibody even when no visible reaction occurs. 

 For example, phenyl-azo-p-benzene-arsonic acid was 

 coupled with horse serum and an antiserum prepared 

 against the complex. The antiserum was allowed to react 

 with the hapten (phenyl-azo-77-benzene-arsonic acid) in a 

 dialysis chamber and the concentration of free hapten 

 which passed through the membrane measured. Most of 

 the hapten was shown to be bound to the antibody. If an 

 antiserum to the corresponding sulphonic acid was used 

 instead of the homologous antiserum no combination 

 occurred and the hapten passed through the membrane 

 until equilibrium was established, showing that the 

 reaction was specific. If the antibody was allowed to 

 react with the antigen before introducing the hapten, 

 no combination with the latter occurred. 



Toxin-antitoxin Reactions. — Toxin and antitoxin com- 

 bine in definite proportions to give a product which is 

 non-toxic. Ehrlich defined the unit of antitoxin as that 

 amount of it which would completely neutralise 100 

 minimal lethal doses (MLD) of toxin. The L+ dose of 

 toxin is that amount of toxin which when mixed with 1 

 unit of antitoxin will kill a 250 g. guinea-pig within 

 ninety-six hours. The L^ dose is that amount of toxin 

 which when mixed with 1 unit of antitoxin will just not 



