THE ZONE PHENOMENON 



203 



What is the relation between the constant-antibody O.R. and the constant- 

 antigen O.K. ■? The obvious expectation would be to find them identical ; but in 

 fact they may be very different. 



Duncan (1932a), working with a crude polysaccharide from a yeast-Kke fungus and the 

 corresponding antiserum, records a constant-antibody O.R. of 1 : 8 and a constant-antigen 

 O.R. of 1 : 64, the latter corresponding to a mixture containing a concentration of antibody 

 eight times greater than the former. Taylor (1933), working with crystalhne egg albumin 

 as an antigen, and a considerable number of dififerent antisera prepared by the injection of 

 rabbits, found that the antibody concentration at the constant- antigen O.R. was approxi- 

 mately 1-65 times the concentration at the constant-antibody O.R. Moreover, ten antisera, 

 tested by both methods, gave figures that did not fluctuate widely about this mean value. 

 It would appear that, for any given system, the constant-antibody O.R., the constant- 

 antigen O.R., and hence the ratio of one to the other, are constant values, though the 

 constant-antibody O.R. may differ widely from the constant- antigen O.R. In different 

 systems, however, the relation of one optimal ratio to the other may vary over a consider- 

 able range. 



To make the relation between the two optimal ratios quite clear, we may consider 

 an actual titration, in which the times of first flocculation are recorded for a series 

 of mixtures of antigen (horse serum) and antibody (rabbit-v.-horse antiserum). 

 The figures, for which we are indebted to our colleague Dr. J. C. Cruickshank, are 

 set out in Table 23. Dilutions of antigen are given in the top row, dilutions of 

 antiserum in the first column. Times are recorded in minutes. Each row corre- 

 sponds to a constant-antibody titration, each column to a constant-antigen titration. 

 The shortest time in each row which corresponds to the constant-antibody O.R. 

 is underlined with a single line. The shortest time recorded in any column corre- 

 sponding to the constant-antigen O.R. is doubly underlined. Where the shortest 

 time in any column is the first recorded figure it is not underlined The blank spaces 

 in the table correspond to tubes in which no visible flocculation occurred during the 

 period of observation. It will be noted that they fall in the zone of gross antigen 

 excess. 



TABLE 23 



Showing the Flocculation Time in Minutes of Difb'erent Mixtures of an Antigen 

 (Horse Serum) with its Homologous Antibody 



