20 INTRODUCTION TO IMMUNOCHEMICAL SPECIFICITY 



Another way of comparing the inhibiting powers of different hap- 

 tens is to use constant concentrations (preferably expressed as molari- 

 ties) of haptens against successive dilutions of the antiserum. In 

 this case, again using the hypothetical haptens H, H', H", and G, 

 we might obtain the sort of result shown in Table 2-3. 



TABLE 2-3 



Precipitation Reaction of Anti-H Antibody and H Antigen, 

 in Presence of Hapten* 



* Symbols as in Table 2-2. 



Instead of trying to find the antibody concentration which is com- 

 pletely inhibited by a given concentration of hapten, or the hapten 

 concentration which will completely inhibit a given concentration of 

 antibody, it is more accurate to measure the amount of precipitate 

 produced under the various conditions, and estimate the amount of 

 hapten which gives just 50 per cent inhibition. 



Statistical Methods 



If several such series of quantitative measurements are carried 

 out, it is possible to obtain a mean (average) estimate of the 50 

 per cent inhibiting dose and, from the standard deviation of this 

 mean, an estimate of its reliability. When such standard errors 

 are calculated they tend to be rather large, for the quantitative 

 precipitin technique is not as reproducible as the measurements of 

 inorganic quantitative analysis or physical chemistry are. For this 

 and a variety of other reasons, standard errors are not usually 

 calculated for such estimates : (a) The necessary determinations would 

 require too great an outlay of the experimenter's time and of an 



