ANTIBODIES I 5 



pure state. Studies of the changes in the composition of blood follow- 

 ing immunization have suggested that antibodies belong to the class 

 of serum proteins called globulins. The distinction between serum 

 albumin and serum globulins was originally based on solubility 

 characteristics in neutral salt solutions (Cohn et al., 1940; Svensson, 

 1941). It is now based more on the observation that in an electric 

 field the serum globulins move more slowly, at alkaline pH (Tiselius, 

 1937). In his classical paper, Tiselius (1937) pointed out that nor- 

 mal serum globulin showed components of at least three different 

 electrophoretic mobilities, and he designated them as alpha, beta, and 

 gamma globulins in order of decreasing mobility. Antibodies, with 

 some possible exceptions, belong to the gamma globulin class. It 

 is this group of globulins that increases most following immunization 

 (TiseHus and Kabat, 1939). 



In man, the rabbit, and many other species, antibodies are found by 

 ultracentrifugal measurements to have the molecular weight charac- 

 teristic of serum globulins, namely about 160,000. In the horse, pig. 



Fig. 1-3. Models of typical antibody molecules with human serum albumin 

 for comparison, (a) Horse anti-pneumococcus antibody; (b) horse antitoxin; 

 (c) rabbit anti-ovalbumin antibody; (d) human anti-pneumococcus antibody; 

 (e) rabbit anti-pneumococcus antibody; (f) human gamma globulin; and (g) 

 human serum albumin. 



