ANTIBODIES II 



15 



-6 -5 -4 -3 -2 



Fig. 2-3. Probability ("error") distribution when standard deviation equals 

 1 and 2. 



The answer is that with simple haptens such as metanih'c acid 

 no visible reaction occurs as a rule. Originally, it was thought that 

 the simple hapten was too small to take part in a precipitin reaction, 

 but we are now inclined to believe that, although size may have 

 something to do with it, the main deficiency of metanilic acid and other 

 simple haptens in this respect is that they have only one point of attach- 

 ment (combining group) for the antibody. Haptens containing two 

 or more combining groups sometimes precipitate with the anti-hapten 

 antibody. 



Nevertheless, the anti-metanilic acid antibody has a strong af- 

 finity for metanilic acid, for it combines with this hapten when it is 

 part of a metanilic acid-protein compound and forms a specific pre- 

 cipitate in the usual way though it does not precipitate metanilic acid 

 from simple solution. The anti-M antibody may even combine with 

 metanilic acid itself without producing a precipitate. 



Inhibition Reactions 



Now it was known that an excess of antigen usually prevents the 

 production of a specific precipitate or greatly diminishes the amount. 

 Clearly it must do this by combining with the antibody, for addition 

 of an imrelated antigen has no such inhibitory efifect. Therefore, 

 Landsteiner reasoned that an excess merely of the hapten, which 



