138 THE PHYSICS OF VIRUSES 



and this will appear later. It is no surprise to anyone who has 

 worked with the serological affinity of viruses that there are 

 many surface antigens. The small-size antigen is perhaps more 

 remarkable. The area of 3 X 10~^^ cm^ or 300 A^ was found by 

 Hutchinson (1952) for the effective antigenic area of monolayers 

 of bovine serum albumin. Quite small molecular units are, 

 therefore, capable of specific combination. Probably four or 



Small Antigen 



Large Antigen 



Diameter 298 A 



Fig. 5.5. Representation of the surface of southern bean mosaic virus as 

 deduced from deuteron and electron bombardment and measurement of the 

 serological affinity. The virus surface is composed of about 200 antigenic 

 units of molecular weight about 15,000, and 1,200 subunits of molecular 

 weight about 6,000. 



five amino-acid side chains in a specifically repeated pattern, 

 throughout the virus surface, form the combining antigenic units. 

 Since there are only 20 amino acids, the possible combinations of 

 four of these is not so enormous a number as the possible protein 

 molecules. This may mean, as has been mentioned before, that a 

 fairly limited number of specific group combinations on anti- 

 bodies exist. 



One byproduct of this type of work results. If the removal 

 of serological affinity requires the inactivation of several hundred 

 molecules, whereas the destruction of infectivity needs only a 



