NATURE OF VIRUSES 9 



The technique is as follows. As pure a preparation as is prac- 

 tical is injected into the serum-producing animal (for example, 

 a rabbit or horse) in three doses a few days apart. At the end 

 of three weeks, the animal is bled. The serum is sei)arated by 

 spinning out the blood cells, and for crude purposes can be used 

 as an antiserum. It is usual to purify the antiserum further 

 by adding specific substances known to be present in the original 

 preparation, for example, plant sap from healthy plants, or 

 bacterial debris from broken-up bacteria in the case of bacterial 

 viruses. These cause precipitation of the antibodies specific to 

 them and leave a more s])ecific antibody to the virus itself. 



Two standard methods of measuring serological combining 

 power, or affinity, are in general use. The precipitin method 

 relies on the observation of a visible precipitate after the addi- 

 tion of antibody to virus; the neutralization of infectivity 

 utilizes the fact that, after combination, the infectivity is re- 

 moved or reduced. 



Virus serology will be taken up in much more detail later. 

 It can be said now that viruses have of the order of 1,000 anti- 

 genic units on their surface; that several different types of unit 

 exist per virus; and that the units differ in size, ranging from 

 the equivalent of eight or ten amino acid side chains, to whole 

 protein molecules. The parts of a bacterial virus which cause 

 attachment to a bacterium do not seem to be active antigens, 

 although this can not be asserted too vigorously. 



Hemagglutination 



An interesting property of a fair number of animal viruses, 

 such as influenza, Newcastle disease, vaccinia, and mum])s, is 

 that of causing red blood cells to clump together, or agglutinate. 

 This is in some cases due to substances called agglutinins, which 

 are capable of separation from the virus, and in other cases 

 due to units in the virus itself. An interesting feature of some 

 of these viruses is the property of self-elution, by which is meant 

 that after a while the virus will remove itself from the red cell 

 and agglutination will cease. After this process has occurred, 

 the red cells will not longer agglutinate. 



