THE SURFACE OF VlRUSEB -'' l-S 



For southern bean mosaic virus p' / p = l.<>8, so the vapov ju'es- 

 sure is 8% higher than ambient. A virus, therefore, will tend to 

 dry even in the presence of moderately high vapor pr^^surqs. It is 

 essentially certain that all the water held by a virus is bound by 

 some kind of moderately strong force not (necessarily) as strong 

 as a covalent bond but of the order of a hydrogen bond in 

 strength. 



Surface Functions of Viruses 



Many of the known properties of viruses are due to their 

 surface. Those of great importance are now listed and briefly 

 described. 



Adsorption. This is best understood for bacterial viruses 

 but is almost certainly a factor for all viruses, with the possible 

 exception of plant viruses. The facts known about adsorption for 

 bacterial viruses will be considered in more detail later in the 

 chapter. 



Serological Behavior. In some manner, the virus protein is 

 able to condition the formation of antibodies in the blood serum 

 of animals. These antibodies are then able to combine specif- 

 ically with the surface virus proteins. This can be studied 

 directly by measuring the precipitate formed, using various 

 means for estimating small amounts of precipitate. Indirect 

 means of various kinds have been devised, of which the most 

 notable is neutralization of infectivity, by which the failure of a 

 preparation to be infective at a certain dilution is taken as 

 evidence that specific combination with antibody has taken 

 place. Another indirect method is complement fixation, which 

 is a remarkably sensitive technique embodying two separate 

 serological processes. Sheep red cells can be dissolved by the joint 

 presence of a thermally unstable substance present in normal 

 serum, complement, and a specific antibody produced in rabbits 

 after injection with sheep red cells. Complement is removed from 

 serum by the combination of an antigen and an antibody. When 

 complement is removed, the red cells are not dissolved. So the 

 presence of specific combination between virus and antibody is 

 inferred from the absence of complement, or the lack of dis- 



