78 AN INTRODUCTION TO THE STUDY OF VIRUSES 
mosaic virus does not remove all the antibodies against tobacco mosaic 
virus. In other words, tobacco mosaic virus contains antigenic 
material that is lacking in aucuba mosaic virus. Similarly, the tests 
with aucuba mosaic virus serum indicate that aucuba mosaic. virus 
contains antigenic material that is absent from tobacco mosaic virus. 
The absorption precipitates show, however, that the extracts of these 
two viruses also contain common antigenic material. Chester suggests 
that the antigenic constitution of tobacco mosaic virus might be 
expressed as XY, that of aucuba mosaic virus as XZ, X being the 
common antigenic fraction, Y the fraction peculiar to tobacco mosaic 
virus, and Z the fraction peculiar to aucuba mosaic virus. 
Serological methods can also be used to show that apparently 
unrelated plant viruses may contain common antigens and so are 
related strains. A good case in point is the relationship shown to exist 
between tobacco mosaic virus and two virus strains affecting cucumber 
known as cucumber viruses 3 and 4. This relationship could not be 
shown in any other way since the respective viruses have no common 
host plant on which cross-immunity tests might be made (Bawden 
and Pirie, 1937). 
Conversely, serological methods will sometimes demonstrate that 
what was thought to be a single virus entity is actually several unrelated 
viruses. This is true of the tobacco necrosis viruses which are 
indistinguishable by biological methods (Bawden, 1941). 
It has been previously pointed out that, as a general rule, a plant 
already infected with a given virus cannot be re-infected with a closely 
related strain of the same virus. With some plant viruses, however, | 
where a large number of strains exist, this rule does not hold good 
with the more distantly related viruses and the cross-immunity test 
breaks down. It has recently been shown (Matthews, 1949) that out 
of a number of strains of potato virus X, some would cross-immunize 
against each other and some would not. The antigenic relationship 
of the pair of viruses that would cross-immunize was closer than 
that of the pair which would not, so that a correlation appears to 
exist between immunizing power and antigenic structure of the viruses 
in question. 
Anaphylaxis | 
In general, when a guinea-pig is injected with an antigen (the 
anaphylactogen), if a period of ten or more days is allowed to elapse 
(the period of incubation), a state of sensitivity to the antigen develops 
