ANTIBODIES II 



27 



substances are found in certain plants (Springer, 1958). One may 

 predict that the number of such serological similarities will grow 

 as the number of individual antigens tested for cross-reactivity 

 increases. 



Combining Groups of Antibody 



The fact that no striking chemical differences between antibodies 

 or between antibody and normal globulin have yet been found sug- 

 gests that the portion of the antibody molecule responsible for its 

 specific combining properties cannot be very large. This idea is sup- 

 ported by the evidence, to be discussed in the next chapter, that the 

 portion of the antigen with which an antibody combines is relatively 

 small, at least compared with the size of a protein molecule. From 

 experiments of Landsteiner and van der Scheer (1938), Campbell and 

 Bulman (1952) computed that the specific combining site of an anti- 

 body is not larger than 700 square angstrom units (700 A^). 



It is believed that van der Waals forces are among the most im- 

 portant in the union between antibody and antigen. Since these are 

 very short range forces, being inversely proportional to the seventh 

 power of the distance, the combining groups of antibody and antigen 

 probably come into intimate contact to produce union as firm as that 

 actually observed. (The free energy change — AF is of the order of 



Fig. 2-11. \^an der Waals outlines of o-, m-, and /'-azobenzenearsonates. 

 (From L. Pauling and H. A. Itano (eds.), 1957, Molecular Structure and Bio- 

 logical Specificity, American Institute of Biological Sciences, Washington, by 

 permission of the editors and publishers). 



