THE SURFACE OF VIRUSES 133 



cell which is not its host cell it is probably active, and one sign of 

 this activity is the formation of antibodies. 



Serological Techniques. Excellent accounts of these exist, and 

 references are given at the end of the chapter. Two features of 

 the technique are important. It is assumed that, as far as possi- 

 ble, an antiserum containing only the specific antibodies to the 

 virus has been produced. This can be achieved by differential 

 precipitation of all but the virus antiserum. 



The two features of importance are (a) appropriate concen- 

 tration relation between antibody and virus, and (b) assay of the 

 amount of virus-antibody combination formed. The former is of 

 importance because as the amount of antigen (virus) is increased 

 relative to antibody the amount of precipitate increases, stays 

 constant, and goes down. Therefore, working in the region of 

 antibody excess must be established or ambiguities can result. 

 Preliminary work has to be done to establish that the proper 

 conditions hold. The second feature, the measurement of the 

 amount of combination between virus and antibody, can be done 

 in several ways. The simplest is to estimate by eye the amount 

 of precipitate formed under favorable lighting conditions for 

 different dilutions of virus and antiserum. A new technique due 

 to Moorhead and Price (1953) raises the sensitivity of this 

 method by using fresh, washed, sheep red cells as an indicator. 

 Any precipitate between virus and antibody will keep the red 

 cells from settling, and thus they form a convenient indicator for 

 the presence of precipitate. 



A quantitative method is to form the precipitate by incuba- 

 tion with antibody for the appropriate time (varying from 3 

 hr to 48 hr depending somewhat on the character of the test being 

 made), centrifuge, remove the supernatant, dissolve the pellet in 

 NaOH, and look for protein absorption at 2,750 A in a 

 spectrophotometer. 



A third method is to employ the loss of infectivity of the 

 virus as a result of antibody combination. This is particularly 

 useful for bacterial viruses where the infectivity assay is direct 

 and easy. Since the question of neutralization of infectivity is of 



