SEROLOGY AND SYSTEMATICS 



73 



5 6 7 8 9 10 11 12 

 Antigen Dilution 



Fig. 5-1. Illustration of the concept of optimal proportions in the precipitin re- 

 action. Note that the greatest amount of interaction occurs at different antigen 

 dilutions for different species. Thus the total area under the curve represents, 

 more accurately, the degree of interaction (Gemeroy, Boyden & De Falco, 1955). 



The technique recommended by Boyden requires a measure- 

 ment of the total area under the curve obtained by measuring the re- 

 action with various antigen dilutions. Relationships are expressed as 

 the per cent area of a particular heterologous reaction compared to 

 the homologous reaction (in a homologous reaction, the antiserum is 

 matched with its original inducing antigens, and the amount of re- 

 action, or curve area, is denoted as 100 per cent). The higher the per- 

 centage of reaction obtained with a particular heterologous reaction 

 the closer would be the presumed serological affinity. 



Despite the improvements in technique such as described 

 above, some workers question the validity of methods based on 

 strictly quantitative reactions. Gell et al. (1960) have pointed out, for 

 example, that when species A gives more precipitate in a heterologous 

 reaction with B than it does with C, this may reflect varying amounts 

 of a single protein which is abundant in B and not in C. How- 

 ever, species A and C may contain several common, non-cross-reacting 

 substances and might reasonably be regarded as more closely related 

 species, although the serological method utilized would obscure this 

 relationship. The actual extent to which such theoretical objec- 

 tions, in practice, detract from the validity of serological data cannot 

 easily be ascertained, but it does appear that such complications are 

 sufficiently probable that every effort should be made to come to 

 grips with some of the more fundamental aspects of the method. 



