THE SURFACE OF VIRUSES 137 



Virus Area 



TMV 1.5 X lO-i^cm^ 



SBMV 3 X 10-1* cm2 and 1 . 5 X IQ-'^ cm- 



T-1 1.5 X 10-13 cm2 



In the case of T-1 ])hage, similar results are found. The area 

 per antigen is given in the table above. 



These results are remarkable in that they indicate that the 

 surface antigenic unit is quite small and also possibly multiple in 

 type. The smallness of the unit gives rise to some speculation as 

 to whether there may not be a rather limited number of possible 

 types of antibody. 



In the case of southern bean mosaic virus, the same ability 

 to precipitate was measured after electron bombardment. The 

 same type of double curve was obtained, again indicating two 

 classes of antigen. It is possible, in principle, to apply the area 

 and volume considerations of densely ionizing and sparsely 

 ionizing particles. In the case of the small antigen, this can be 

 done roughly, and the resulting sensitive volume is found to 

 be about 7 X 10~-^ cm^ with a very roughly spherical shape. The 

 molecular weight equivalent is about 6,000. The area of each 

 antigen is about 3 X 10~^^ cm^, and of the whole dry virus it is 

 3.5 X 10~^^ cm^, so there are presumably 1,700 such small 

 antigens. The large antigen cannot be analyzed so well, but 

 by pushing the data a little, a molecular weight equivalent of 

 15,000 is obtained, with a roughly spherical shape and an area 

 per antigen of roughly 1.5 X 10~^^ cm^. Assuming these cover the 

 virus surface, there are about 200 large antigens. 



The resulting picture of the virus is shown in Fig. 5.5. The 

 large antigens alone are drawn in, but the subdivision of each 

 into smaller units is also shown for one case. 



The warnings contained in Chapter 1 must be repeated here. 

 Such pictures and such models are necessarily inferential. For 

 instance, the second antigen may not prove to be part of the 

 virus itself. Other evidence should support them before they 

 can be taken as valid. A certain amount of such evidence exists 



