no INTRODUCTION TO EVOLUTION 



them are found in the blood serum of the animal which produced them. 

 Such an antibody-containing serum is called an antiserum. Thus, if some 

 of the blood serum from a rabbit inoculated as described above is removed 

 and mixed with horse serum, the antibodies will react with the horse se- 

 rum to form a white precipitate. Hence antibodies of this kind are called 

 precipitins, and the serological test making use of them is called the 

 precipitin test. 



In most of the precipitin tests of interest to the study of evolution, do- 

 mestic rabbits have been used as antibody producers, partly as a matter of 

 convenience, partly because rabbits produce antibodies more readily 

 than do some other laboratory animals. 



Homology of Serum Proteins 



As an example of the application of the precipitin test we may consider 

 the results of tests performed with antibodies formed by rabbits inocu- 

 lated with human serum (Fig. 6.1). If the inoculation is properly done the 

 rabbit will form antibodies against the human serum. Blood is then with- 

 drawn from the rabbit and the serum is removed from the blood cells. 

 The antiserum so obtained will be found to contain antibodies specific for 

 human serum. As shown in the figure, if a little of the antiserum is mixed 

 in a test tube with human serum a white precipitate will form and will 

 settle to the bottom of the tube. 



The test is continued by mixing in another test tube some of the anti- 

 serum, containing antibodies against human serum, with some chimpanzee 

 serum. Will antibodies formed against human serum react with chimpan- 

 zee serum? Reaction will depend upon whether or not the proteins of 

 chimpanzee serum are sufficiently similar in chemical structure to those of 

 human serum so that antibodies formed against the one will react with the 

 other. As the diagram shows, most tests performed as indicated, without 

 additional refinements, fail to distinguish chimpanzee serum from human 

 serum, the same amount of precipitate being formed in both tubes. In 

 other words, chimpanzee serum and human serum seem almost exactly 

 ahke in chemical structure. 



The test shown in the diagram continues by the mixing of a third sample 

 of rabbit antiserum, containing antibodies against human serum, with ba- 

 boon serum. Will reaction occur in this tube? Again the answer will de- 

 pend upon whether or not baboon serum is sufficiently similar to human 

 serum so that antibodies formed against the latter will react with the for- 

 mer. In agreement with tests which have been performed, the figure indi- 



