6 INTRODUCTION TO IMMUNOCHEMICAL SPECIFICITY 



cow, and perhaps other species, anti-pneumococcus antibodies, for 

 example, have a molecular weight of about 900,000. Certain anti- 

 bodies in human blood seem to have molecular weights somewhere 

 between these values (discussion in Boyd, 1956). From ultracentri- 

 fugal sedimentation constants, diffusion constants, and Perrin's 

 (1936) relation between the "frictional ratio" and axial ratio of a 

 prolate spheroid, the shape of protein molecules may be calculated. 

 Photographs of models of typical antibody molecules, with human 

 serum albumin for comparison, are shown in Fig. 1-3. 



Actual photographs of antibody molecules, taken with the electron 

 microscope, reveal, as far as the still inadequate resolving power 

 of this instrument allows, a striking similarity to the models shown 

 in Fig. 1-3 (see Fig. 1-4). Also of interest in the photograph 

 is the apparent heterogeneity in size. 



Fig. 1-4. Electron micrographs of rabbit antibody molecules. (Photograph 

 by Dr. C. E. Hall.) 



Specificity 



It is a very old observation that immunity is specific. A child who 

 has recovered from whooping cough is very unlikely to get this disease 

 again in the immediate future, but his resistance to measles is not 



