74 AN INTRODUCTION TO THE STUDY OF VIRUSES 
mosaic virus producing a green mottle cannot be infected with a 
related virus which gives rise to a yellow mottle. 
The explanation of this type of immunity seems to be that a cell 
already fully parasitized by one virus cannot be invaded by another 
virus of a related type. It can, however, be infected by other and 
unrelated viruses. This phenomenon, incidentally, provides a useful 
method for identifying related viruses. 
Nevertheless, plant viruses are potent antigens and when inoculated, 
in a fairly pure state, into a suitable animal, generally a rabbit, give 
rise to their specific antisera. Since this fact was first demonstrated 
in 1928 by Purdy (Beale) a great deal of intensive study on the serology 
of the plant viruses has been carried out. 
In preparing antisera to plant viruses, the rabbit is the most suitable 
animal and the following is a brief description of the method. As 
already pointed out, it is important to have the virus sample as free 
as possible from plant proteins which are also antigens though not 
particularly good ones. This is usually done by precipitation with 
ammonium sulphate, followed by dialysis to remove excess of salt. 
(Chapter V on purification of viruses.) The following method is 
suitable for fairly pure virus preparations but for impure materials 
the interperitoneal injection should be used. 
The rabbit used for preparing antisera should be large (about 4 lb) 
and should preferably have large ears with prominent veins. A 1-ml 
hypodermic syringe with a thin needle (about size 14) is used. The 
syringe is filled and air bubbles are expelled with the tip of the syringe 
held upright. The injection is made into the vein which runs along 
the upper surface of the ear, parallel to the hind edge and about j-in. 
to #-in. from it (Plate XVI, lower photograph). It is useless to try 
to use the other veins, even though they may appear larger. 
If a rabbit is going to receive a scries of injections, it is preferable 
to give the first near the tip of the ear and each later injection 
successively closer to the base. 
The hair on the ear is smoothed down by wiping with cotton wool 
dipped in alcohol, or the ear may be shaved. The tip of the needle is 
:nserted into the vein in the direction of the base of the ear, and 1 ml 
of the virus is injected. If the tip of the needle is in the vein, the liquid 
will flow in smoothly. The needle is withdrawn gently, slight pressure 
being maintained over the point of entry with the thumb of the left 
hand. 
About two weeks after injection, the rabbit is bled from the other 
