THE PHYSICS OF VIRUSES 



tivity. It seems as though something involving arrangement, 

 rather than chemical content, is of great importance. 



Virus Serology 



Viruses are excellent antigens, and, accordingly, the sero- 

 logical behavior of viruses gives us an excellent method of study 

 of viruses, particularly as regards their surface jjroperties. To 

 illustrate the potency of viruses as antigens, a preparation of 

 infectious sap of a plant virus, which has in no sense been 

 thoroughly purified, is antigenic primarily because of the virus 

 and not because of the unremoved cell constituents also present. 



It is unfortunate that we do not yet know how antibodies are 

 produced in response to antigens. It is not a simple physical 

 process, for antibodies can continue to be formed after the 

 original antigen molecules have disajjpeared. Yet there seems 

 to be an underlying simplicity about the end results, for the 

 result of injecting antigen into an animal is to produce mole- 

 cules in the blood serum which are clearly related to the original 

 antigen molecules and have for them a strong and specific 

 affinity. Both the affinity and its specificity can be used in virus 

 study, and a very great branch of virus research proceeds along 

 serological lines. 



