COMPARATIVE PHYSIOLOGY AND BIOCHEMISTRY 



guinea pig is then said to be immunized to the kind of blood which was 

 injected. The precipitation reaction will occur in a test tube as well as in 

 the bloodstream. Thus, if one prepares an antiserum from an immunized 

 animal and adds to it a few drops of antigenic serum, a precipitate will 

 be formed. This can be measured by two principal methods. The first of 

 these is the ring test method ( Figure 22 ) . A small quantity of undiluted 

 antiserum is placed in a test tube, and diluted antigenic serum is then 

 carefully layered over it. A ring of precipitate then forms at the interface 

 between the two sera. The greatest dilution of the antigenic serum at 

 which a ring is obtained gives a measure of the strength of the reaction, 

 with a high dilution corresponding to a strong reaction. If, however, the 

 two sera are mixed, the precipitate will make the solution turbid, and the 

 photometric measurement of the absorption of light gives an excellent 

 measurement of the strength of the reaction. 



Such antigen-antibody reactions are highly specific. That is, an antibody 

 which precipitates the blood of one species is generally ineffective against 

 the bloods of other species. Yet the specificity is not complete, for serum 

 immunized against the blood of any species A will precipitate the bloods 

 of species related to A, but in ever-decreasing degrees as the relationship 

 grows more distant. For example, a guinea pig may be immunized with 



A 



Figure 23. Relationships of Crypto- 

 branchus (C), Amphiuma (A), Siren 

 (S), AND Nectiinis (N) as Deter- 

 mined BY Serological Tests. The 

 diagram shows the proportional "dis- 

 tances" between the species. (From 

 Boyden and Noble, Am. Museum 

 Novitates, No. 606, 1933. ) 



N 



57 



