SEROLOGY OF VIRUSES 79 
in this animal. When the same antigen is injected intravenously into 
an animal thus sensitized, a severe or fatal shock (anaphylactic shock) 
usually results: This anaphylactic reaction is so highly specific and can 
be induced by such small amounts of antigen that it is often utilized 
for the identification of substances where qualitative chemical methods 
are difficult (Beale and Seegal, 1941). This method was investigated 
by Chester (1936°) who used the Schultz-Dale technique. This 
technique is based on the fact that if a virgin female guinea-pig is 
properly sensitized with a protein, its excised uterine horns, when 
placed in an isotonic bath, will exhibit contraction when the specific 
protein used in sensitation is added to the bath. Such contraction 
can be conveniently recorded on a kynograph drum. Chester applied 
this method to several plant viruses including that of tobacco mosaic, 
but found that none of the viruses tested gave anaphylactic reactions. 
On the other hand, proteins from healthy tobacco and other plants 
were highly anaphylactogenic. By this means, then, it was possible 
to show that apparently pure preparations of tobacco mosaic virus 
protein actually contained a quantity of normal plant proteins. 
The serological reactions can also be used as a quantitative measure 
of plant viruses. There are two methods, the precipitation end-point 
and the optimal precipitation point. 
Beale (1934) first showed that the precipitation end-point method 
could be used for estimating virus concentration. The antigen she 
used was tobacco mosaic virus and she diluted the antigens until a 
point was reached at which a visible precipitate was just visible when 
mixed with antiserum. A quantitative relation was shown to exist 
between the antigenic content and the active virus concentration of 
eleven separate extracts compared in eight different pairs. 
Beale determined the antigenic content by titrating the virus 
against a constant amount of antiserum and checked the infectivity 
by a biological method of plant inoculation. In the optimal precipi- 
tation point method the antigen is used at a constant dilution and the 
concentration of the antiserum varied. Bawden (1935) used this 
method in determining the virus content of various samples of the 
potato X virus. He used a semi-purified virus, one from which most 
of the plant proteins had been removed. The flocculation experiments 
were carried out in 7 mm-thick walled glass tubes, 0-9 c.c. of antigen 
at constant dilution being added to a series of tubes, each containing 
0'9 c.c. of antiserum at varying dilutions ranging from } to ss. The 
tubes were then placed immediately in a water-bath at 50°C so that 
6a—(T.502) 
